CaRLO

Notes: A framework for estimating heating and expected temperature rise in current carrying molecular junctions is described. Our approach is based on applying the Redfield approximation to a tight binding model for the molecular bridge supplemented by coupling to a phonon bath. This model, used previously to study thermal relaxation effects on electron transfer and conduction in molecular junctions, is extended and used to evaluate the fraction of available energy, i.e., of the potential drop, that is released as heat on the molecular bridge. Classical heat conduction theory is then applied to estimate the expected temperature rise. For a reasonable choice of molecular parameters and for junctions carrying currents in the nA range, we find the temperature rise to be a modest few degrees. It is argued, however, that using classical theory to describe heat transport away from the junction may underestimate the heating effect. © 2002 American Institute of Physics.

Notes: The selective separation of ethanol from ethanol–water mixtures by ultrasonic atomization has been reported recently by Sato, Matsuura, and Fujii [J. Chem. Phys. 114, 2382 (2001)]. In that work, experimental data were reported that confirmed the generation of an ethanol-rich droplet mist and attempted to explain the selective separation in terms of parametric decay instability of the capillary wave formed during sonication. In the present work, an alternate mechanism based on the conjunction theory has been postulated for the process of ultrasonic atomization. This mechanism involves the formation of cavitating bubbles in the liquid during sonication and their eventual collapse at the liquid surface into a cloud of microbubbles that moves upwards in a capillary fountain jet. The selective separation of alcohols has been explained as a corollary effect of the physical mechanism resulting in a surface excess of alcohol molecules formed at the surface of the microbubbles. The alcohol molecules vaporize into the microbubbles and release an alcohol-rich mist on their collapse in regions of high accumulation of acoustic energy.

Notes: No reliable structural data have been reported on ice II under pressure, earlier work in the literature relating either to samples recovered to ambient pressure or the helium hydrate that is formed when helium is used as the pressurizing medium. We report structural refinements of helium-free ice II at three points in the phase's region of stability. The structural differences from the helium-affected structure are significant, and can be related to the mainly repulsive interaction between the helium and both the oxygen and hydrogen atoms of the ice framework. These repulsions explain, among other changes, the different behaviors of the a (expansion) and c (contraction) lattice parameters, and the change in compressibility on the inclusion of helium. © 2002 American Institute of Physics.

Notes: We analyze the layering structure for the free liquid surface observed at low temperature in simple fluid models, with pair interaction potentials. The relationship of the surface layers with the Fisher–Widom line is discussed, by direct comparison of Monte Carlo simulation results for the liquid–vapor density profiles and the pair distribution function of the bulk liquid. Also we study the role of the capillary waves in the damping of the surface oscillatory profiles, with Monte Carlo simulations for different transverse areas, and through the theoretical scaling forms using the values of the surface tension given by our simulations. The main conclusion is that the dependence of the surface layering with the temperature is dominated by the capillary waves, even at the small transverse sizes typically used in computer simulations. In contrast, the Fisher–Widom line seems to be of minor importance for the amplitude of the layering. © 2002 American Institute of Physics.

Notes: A c(2×4) LEED pattern is observed for methylidyne (CH) chemisorbed on Pt{110}(1×2) at a saturation coverage of 0.25 ML. Density functional calculations reveal that methylidyne is preferentially adsorbed in the fcc three-fold hollow site on the {111} microfacet of the reconstructed surface. A structure for the ordered overlayer is thus proposed, and both through-metal and through-space interactions are considered as possible causes for this unexpected long-range coherence. We argue that entropic effects may be implicated. © 2002 American Institute of Physics.

Notes: We used the energy-selected-photoelectron photoion coincidence (ESPEPICO) method to study site-specific fragmentation caused by C:1s photoionization of 1,1,1-trifluoro-2-propanol-d1 [CF3CD(OH)CH3, TFIP-d1] on a Si(100) surface. High-resolution electron energy loss spectroscopy showed that TFIP-d1 is dissociatively chemisorbed like (CF3)(CH3)CDO–Si(100), and different chemical shifts at the three carbon sites were observed by photoelectron spectroscopy. The site-specific fragmentation evident in the ESPEPICO spectra of the sub-monolayer at room temperature indicates that the TFIP-d1 there has an O–Si bond oriented in the trans position with respect to the C–CF3 bond. Here we discuss the fragmentation processes in light of the results obtained with the ESPEPICO method and the Auger-electron photoion coincidence method. © 2002 American Institute of Physics.

Notes: We investigate the formation dynamics of self-assembled polyelectrolyte multilayers on glass substrates by in situ and ex situ second harmonic generation (SHG) measurements and atomic force microscopy (AFM). The time dependence of the SHG signal during the adsorption process is attributed to a time dependent surface potential of the polyelectrolyte film. The dynamics can be quantitatively understood using a random sequential adsorption (RSA) model for the buildup of a film consisting of polyelectrolyte disks with polydisperse sizes. Differences between wet and dry films are also investigated. © 2002 American Institute of Physics.

Notes: The interaction between a polymer segment and an oil–water interface is represented by an asymmetric square-well potential where the well-depth on one side reflects water–polymer and the well depth on the other side reflects oil–polymer interactions. The polymer is represented by a Gaussian chain. The polymer's density distribution is calculated along a coordinate perpendicular to the interface. Results are obtained as a function of the well width, the well depth and its asymmetry and, most important, the polymer's length. For a symmetric well, the distribution shows a strong maximum at the interface provided that the polymer is sufficiently long. For an asymmetric well, the polymer is also strongly adsorbed at the interface provided that the polymer is sufficiently long and provided that the larger well-depth does not exceed a critical value that depends on the smaller well-depth. The calculations are in substantial agreement with experimental results that indicate nearly irreversible adsorption of long-chain molecules at an oil–water interface. © 2002 American Institute of Physics.

Notes: We explore the electrical characteristics of disordered films of strongly coupled, molecularly-linked gold nanoparticles (NPs). dc conductivity vs temperature (g vs T) measurements exhibit features that can track a number of competing transport mechanisms. Films with fewer than 6 layers show clear signatures of both activated tunneling and thermionic emission. Our linked NPs admit locally metallic transport, likely through strong quantum interactions, and at room temperature, films with 6 or more layers exhibit a transition to metallic dominated behavior. Observed g vs T dependencies have been modeled treating arrays as disordered resistor networks and using an effective medium approximation (EMA). Our results show that percolation phenomena can play critical roles in transport through NP films, particularly near metal–insulator transitions. © 2002 American Institute of Physics.

Notes: A microscopic expression for the divergence of the stress tensor is derived for inhomogeneous suspensions of rigid colloidal particles of arbitrary shape. This expression is valid for arbitrary large gradients in the shear rate, concentration, and orientational order parameters. The corresponding Navier–Stokes equation is a necessary ingredient to describe phenomena like shear-banding, phase separation kinetics, and phase coexistence under shear flow conditions. © 2002 American Institute of Physics.

Notes: In this work, we present a simple approach for devising order parameters (OPs) for atomic systems based on pattern recognition techniques. It exploits the fact that all crystalline substances are characterized by a unique "signature" cell (SC) which is constructed using a central atom and its nearest NSC neighbors in a given crystal. The algorithm measures the local degree of similarity between a SC and the system to be analyzed. The best fit of a SC to NSC atoms surrounding a given atom in the system is determined by maximizing a fictitious energy of binding among those atoms and the SC atoms. The fictitious potential energy is designed to give maximum attractive energy for maximum overlap. The maximum binding energy of interaction attained in this process is used as a measure of similarity between the crystal structure and the system (i.e., as an OP). The proposed method provides a unified and intuitive approach for constructing relevant OPs for a given system. We used these OPs to characterize the order of different phases in the Lennard-Jones system and in a model silicon system. It is shown that these novel OPs give a more complete description and a better understanding of the structural order in amorphous silicon than conventional OPs. © 2002 American Institute of Physics.

Notes: CO adsorption on small cationic, neutral, and anionic Aun (n=1–6) clusters has been investigated using density functional theory in the generalized gradient approximation. Among various possible CO adsorption sites, the on-top (one-fold coordinated) is found to be the most favorable one, irrespective of the charge state of the cluster. In addition, planar structures are preferred by both the bare and the CO-adsorbed clusters. The adsorption energies of CO on the cationic clusters are generally greater than those on the neutral and anionic complexes, and decrease with size. The adsorption energies on the anions, instead, increase with cluster size and reach a local maximum at Au5CO−, in agreement with recent experiment. The differences in adsorption energies for the different charge states decrease with increasing cluster size. © 2002 American Institute of Physics.

Notes: An off-lattice Monte Carlo simulation using a coarse-grained bead-spring model was developed to analyze nanoconfined polymer films. The beads in the polymer chains are connected via finitely extensible, nonlinear elastic springs. In addition to nonpolar van der Waals interactions, functional endbeads have a short-range exponential interaction characteristic of endgroup coupling. Our simulated results qualitatively agree with experimental data for perfluoropolyether molecules with functional endgroups in ultrathin films. The chains exhibit an oblate conformation near the wall, but recover a spherical shape as they move farther away from it. The density profile of functional endbeads as a function of distance from the wall shows characteristic oscillation originating from endbead coupling and orientation near the wall. We examined the molecular layering of films via the analysis of an anisotropic radius of gyration for the chains, which is affected by the endgroup interactions. © 2002 American Institute of Physics.

Notes: The Dynamic Lattice Liquid (DLL) model has been implemented as a simulation algorithm for two-dimensional (2-D) polymeric systems. The model can work with density factor ρ=1 and became a basis for a parallel algorithm, which takes into account coincidences of elementary molecular motion attempts resulting in local cooperative structural transformations. Systematic studies of athermal polymer systems on the two-dimensional triangular lattice are presented for various concentrations of linear polymers with respect to an explicitly considered solvent. Both static and dynamic properties are characterized. The simulation results reflect molecular packing and other properties of monomolecular polymer layers, which can be relevant to these obtained in some thin film formation techniques. © 2002 American Institute of Physics.

Notes: Liner and nonlinear-optical properties of oligothiophenes self-assembled monolayers (SAM) are studied by optical second harmonic interference technique. Linear polarizability and first hyperpolarizability as functions of the number of thiophene rings are calculated using a self-consistent approach for linear and nonlinear optical properties. Both of these dependencies can be described by a power law; a nonzero first hyperpolarizability points to the asymmetry of delocalized electron system that might be due to an adsorption of thiophene molecules on a metal substrate. The influence of a dipole–dipole interaction of molecules arranged in the SAM is discussed. © 2002 American Institute of Physics.

Notes: We extend Teixeira and Telo da Gama's density-functional approach [J. Phys.: Condens. Matter 3, 111 (1991)] to study the vapor–liquid phase equilibria and planar interfacial properties of quadrupolar fluids. The density profile, surface ordering, surface polarization, and surface tension of quadrupolar fluids are evaluated. Particular attention is given to the temperature and quadrupole-strength dependence. It is found that the interfacial molecular ordering can arise entirely due to the quadrupole–quadrupole interaction. It is also found that the bulk-phase properties of the quadrupolar fluids satisfy the law of correspondence states. The reduced surface potential and surface tension can be well correlated by power laws of the scaled temperature τ≡1−T/Tc, where Tc is the critical temperature of the fluids. © 2002 American Institute of Physics.

Notes: We investigate the effect of polymer concentration on the diffusion and localization (entropic trapping) of linear polymer chains in a two-dimensional model system of small obstacles and large pores. Three distinct regimes are identified: the entropic trapping regime, a reptation regime where the larger pores are polymer saturated and the untrapped polymers reptate, and finally a crowding regime where intermolecular interactions dominate. In this model system, the entropic trapping, reptation, and crowding mechanisms compete and lead to a characteristic maximum in the diffusion coefficient for intermediate polymer concentrations. © 2002 American Institute of Physics.

Notes: Brownian dynamics (BD) simulations of a linear freely jointed bead–rod polymer chain with excluded volume (EV) interaction have been performed under elongational flow with and without the use of fluctuating hydrodynamic interactions (HI). The dependence of the chain size, shape and intrinsic elongational viscosity on the elongational rate cursive-epsilon(overdot) are reported. A sharp coil–stretch transition is observed when cursive-epsilon(overdot) exceeds a critical value, cursive-epsilon(overdot)c. The inclusion of the HI leads to a shift in the coil–stretch transition to higher flow values. Chain deformation due to elongational flow is observed to first consist of the alignment of the chain with the direction of flow without significant chain extension followed by additional alignment of the bond vectors with the flow direction and chain extension as flow rate is increased further. The distribution function for the chain's radius of gyration becomes significantly broader within the transition region which implies an increase in fluctuations in the chain size in this region. The structure factors parallel and perpendicular to the flow direction illustrate different elongational rate dependencies. At high rates, the structure factor in the direction of the flow exhibits an oscillating dependence which corresponds to the theoretically predicted shape for a rigid-rod model. The mean squared orientation of each bond within the chain with respect to the flow direction as function of bond number is nearly parabolic in shape with the highest degree of orientation found within the chain's interior. The dependence of the critical elongational rate, cursive-epsilon(overdot)c, on the chain length, N, is observed to be cursive-epsilon(overdot)c∼N−1.96 when hydrodynamic interactions are not employed and cursive-epsilon(overdot)c∼N−1.55 when they are invoked. These scaling exponents agree well with those obtained in previous BD simulations of bead-FENE (i.e., finitely extensible nonlinear elastic) spring chains as well as with the theoretical predictions of cursive-epsilon(overdot)c∼N−2 and cursive-epsilon(overdot)c∼N−1.5 without and with hydrodynamic interactions based on the Rouse and Zimm models, respectively. © 2002 American Institute of Physics.

Notes: We study the process of charged polymer translocation, driven by an external electric potential, through a narrow pore in a membrane. We assume that the number of polymer segments, m, having passed the entrance pore mouth, is a slow variable governing the translocation process. Outside the pore the probability that there is an end segment at the entrance pore mouth, is taken as the relevant parameter. In particular we derive an expression for the free energy as a function of m, F(m). F(m) is used in the Smoluchowski equation in order to obtain the flux of polymers through the pore. In the low voltage regime we find a thresholdlike behavior and exponential dependence on voltage. Above this regime the flux depends linearly on the applied voltage. At very high voltages the process is diffusion limited and the flux saturates to a constant value. The model accounts for all features of the recent experiments by Henrickson et al. [Phys. Rev. Lett. 85, 3057 (2000)] for the flux of DNA molecules through an α-hemolysin pore as a function of applied voltage. © 2002 American Institute of Physics.

Notes: Second-order quantized Hamiltonian dynamics (QHD-2) is mapped onto classical mechanics by doubling the dimensionality. The mapping establishes the classical canonical structure for QHD-2 and permits its application to problems showing zero-point energy and tunneling via a standard molecular dynamics simulation, without modifying the simulation algorithms, by introducing new potentials for the extra variables. The mapping is applied to the family of Gaussian approximations, including frozen and thawed Gaussians, which are special cases of QHD-2. The mapping simplifies numerous applications of Gaussians to simulations of spectral intensities and line shapes, nonadiabatic and other quantum phenomena. The analysis shows that frozen Gaussians conserve the total energy, while thawed Gaussians do not, unless an additional term is introduced to the equation of motion for the thawed Gaussian momentum. The classical mapping of QHD-2 is illustrated by tunneling and zero-point energy effects in the harmonic oscillator, cubic and double-well potential, and the Morse oscillator representing the O–H stretch of the SPC-F water model. © 2002 American Institute of Physics.

Notes: We present a method for computing intermolecular energies of large molecules based on a suitable fragmentation scheme, which allows one to express the complete interaction energy as a sum of interaction energies between pairs of fragments. The main advantage consists in the possibility of using standard ab initio quantum methods to evaluate the fragment energies. For the 4-n-pentyl-4′-cyanobiphenyl (5CB) dimer, the present results indicate that the most favorite arrangement corresponds to an antiparallel side-by-side geometry with a stabilization energy of about 16 kcal/mol. It is shown that, by the present method, the interaction energy of the 5CB dimer can be evaluated for all geometrical conformations and, in principle, it can be used for bulk simulations. © 2002 American Institute of Physics.

Notes: We have measured both the static and dynamic structure factors of a single dendrimer with small-angle x-ray scattering (SAXS) and neutron spin-echo spectroscopy under good solvent conditions with the aim of finding a consistent correlation between the structural properties of dendrimers and their dynamic behavior. The samples under investigation were star-burst polyamidoamine dendrimers with generations g=0 to 8 in dilute methanol solutions. A model independent approach employing inverse Fourier transformation and square root deconvolution methods has been used to analyze the SAXS data to obtain the pair distance distribution function p(r) and the radial excess electron density profile Δρ(r). In addition, we formulated a model that takes both the colloidal (globular, compact shape with form polydispersity or fuzzy surface) as well as the loose, polymeric (self-avoiding random walk) character of dendrimers into account. With this model we were able to describe the spectra of all dendrimer generations consistently. Parameters discussed as a function of the dendrimer generation are, among others, the correlation length of the density fluctuations (blob radius) ξ, the radius of gyration Rg, the sphere radius Rs, the form polydispersity σs or analogously, the width of the fuzzy surface region 2σf. Both the model-independent approach and the model fits reveal that at least down to the third generation the dendrimers exhibit a rather compact, globular shape. These findings are in agreement with the dynamic results obtained by NSE spectroscopy which probes length scales both larger and much smaller than the dimension of a single dendrimer. The method reveals that the dynamics throughout is dominated by the center-of-mass diffusion—the internal dynamics is suppressed. The diffusion coefficients obtained are close to the values calculated from the Stokes–Einstein relation using the sphere radius Rs determined from the SAXS spectra. Dynamically, the dendrimers behave like "hard", solid spheres. © 2002 American Institute of Physics.

Notes: In this paper we present a new approach to simulation methods for classical statistical mechanics relying on a field-theoretical formalism. It is based on applying the complex Hubbard–Stratonovich transformation to the canonical and grand-canonical partition function, which allows one to reexpress their particle representation in terms of a functional integral over a fluctuating auxiliary field. The thermodynamic averages from the resulting field representations can then be calculated with a conventional Monte Carlo algorithm. We explored the applicability of the auxiliary field methodology for both the canonical and grand-canonical ensemble using a system of particles interacting through a purely repulsive Gaussian pair potential in a broad range of external parameters. In the grand-canonical case this technique represents an alternative to standard grand-canonical Monte Carlo methods. Generally providing a framework for simulating classical particle systems within a continuum formalism can be useful for multiscale modeling where the field or continuum description naturally appears within quantum mechanics on smaller length scales and within classical mechanics on larger ones.© 2002 American Institute of Physics.

Notes: New path integral Monte Carlo constant volume specific heat (CV) estimators are presented that improve upon the thermodynamic, virial, and centroid virial CV estimators via a free particle projection. These projected estimators significantly reduce the numerical noise of the traditional estimators. The new projected thermodynamic estimator has particular advantages when derivatives of the potential are expensive to evaluate. A double virial estimator is derived for real space path integrals and comparisons are made to it. The centroid virial estimators are found to be significantly better than the noncentroid virial estimators. © 2002 American Institute of Physics.

Notes: A one-electron pseudopotential hole–particle formalism is implemented to investigate excitations in xenon molecules and clusters. Within this framework, averaged relativistic electron-Xe and electron-Xe+ pseudopotentials are determined to incorporate the excited particle contributions. A consistent hybrid scheme for spin–orbit coupling is developed, involving an atoms-in-molecules type approximation for the hole and a pseudopotential operator for the particle. The reliability of the one-electron pseudopotential scheme is first checked on the atomic spectrum of xenon and the transferability to high excited states is demonstrated. The molecular behavior of the formalism is also investigated by determining the potential energy curves of the lowest excimer states of Xe2*. The spectroscopic constants (De, ωe, and ωexe, respectively) are found to be 4173, 108, and 1.17 cm−1 for state (1)0u−(6s 3P2), 4197, 109, and 1.11 cm−1 for state (1)1u(6s 3P2), and 4250, 107, and 1.14 cm−1 for state (1)0u+(6s 3P2). © 2002 American Institute of Physics.

Notes: We construct exchange energy functionals that depend on the non-interacting square kinetic energy density τ2(r)=<fraction SHAPE="CASE">14∑k|Δφk(r)|2 rather than the electron density. Since τ2 is a non-local functional of the electron density, it may describe non-local effects beyond standard generalized gradient approximation models. These effects may be essential in dealing with such a non-local quantity as the exchange energy. The problem of v-representability of τ2 for a slightly perturbed electron gas and Coulomb systems is discussed. A gradient expansion technique that permits construction of gradient-corrected functionals of τ2 is developed, and the performance of the new exchange functionals is analyzed. © 2002 American Institute of Physics.

Notes: Spectral, field, and temperature dependencies of steady-state photoconductivity were studied in thin films of pure phenyl substituted copolymeric polyphenylenevinylene (PhPPV) and PhPPV doped by trinitrofluorenone. The films were sandwiched between indium tin oxide (ITO) and aluminum as well as between two aluminum contacts. Typical surface extrinsic carrier photogeneration was observed in ITO/PhPPV/Al samples with positively biased ITO while the intrinsic photoconductivity dominated in Al/PhPPV/Al samples and ITO/PhPPV/Al sandwiches with negatively biased ITO. Charge carrier photogeneration in both doped and undoped PhPPV at higher photon excess energies was found to be in good agreement with predictions of the hot exciton dissociation model. At low excess photon energy of the inducing light, carrier photogeneration is ascribed to dissociation of optical excitations on charge transfer centers which are either dopant molecules in the doped samples or intrinsic defects in the pure material. © 2002 American Institute of Physics.

Notes: Athermal, tethered chains are modeled with density functional (DFT) theory for both the explicit solvent and continuum solvent cases. The structure of DFT is shown to reduce to self-consistent-field theory in the incompressible limit where there is symmetry between solvent and monomer, and to single-chain-mean-field (SCMF) theory in the continuum solvent limit. We show that by careful selection of the reference and ideal systems in DFT theory, self-consistent numerical solutions can be obtained, thereby avoiding the single chain Monte Carlo simulation in SCMF theory. On long length scales, excellent agreement is seen between the simplified DFT theory and molecular dynamics simulations of both continuum solvents and explicit-molecule solvents. In order to describe the structure of the polymer and solvent near the surface it is necessary to include compressibility effects and the nonlocality of the field. © 2002 American Institute of Physics.

Notes: The adiabatic ionization potential of methylene has been determined to be 83772±3 cm−1 from a rotationally resolved photoelectron spectroscopic study of the CH2+ X˜ 2A1 (0,0,0)←CH2 X˜ 3B1(0,0,0) transition. This value was used to determine thermochemical quantities such as the 0 K dissociation energy of the ketene cation in CO and CH2+ D0(CH2(Double Bond)CO+)=33202±7 cm−1, the 0 K dissociation energy of the methyl radical D0(CH2–H)=38179±49 cm−1, the 0 K dissociation threshold of methane in CH2 and H2 D0(CH2–H2)=38232±50 cm−1 and the 0 K enthalpy of formation of CH2 ΔfH(minus sign in circle)(CH2,T=0 K)=390.73±0.66 kJ mol−1. © 2002 American Institute of Physics.

Notes: The pulsed field gradient nuclear magnetic resonance method is applied to study self-diffusion of ethane in beds of zeolite NaX for displacements which are orders of magnitude larger than the size of individual crystals. Comparison of the measured diffusivities with those calculated using simple gas kinetic theory indicates that for the same bed of NaX crystals the apparent tortuosity factor in the Knudsen regime is significantly larger than that in the bulk regime. This finding is tentatively attributed to the more pronounced geometrical trapping by surface imperfections in the Knudsen than in the bulk regime. Tortuosity factors, which are much larger in the Knudsen regime than in the bulk regime, were also recently obtained by dynamic Monte Carlo simulation of gas diffusion in various porous systems. © 2002 American Institute of Physics.

Notes: The proton tunneling reaction in malonaldehyde at low temperatures is investigated. The principal aim of this study is to find the optimal tunneling path at 0 K in the framework of the semiclassical theory with a global optimization method. An amount of 11366 ab inito points was determined in the reaction swath (i.e., the conformational space enclosed by the minima and the transition state) of malonaldehyde. With a simulated annealing approach, the path with the smallest integral of the imaginary action through the swath from minimum to minimum was determined. Surprisingly the optimal tunneling path was found to be quite far off the large curvature tunneling path [i.e., the straight connection of the two minima large-current tunneling (LCT path)]. At the beginning, it is following the minimum energy path (MEP) (i.e. the path with the lowest energy connecting the two minima and passing through the transition state), and then it is describing a curved path through the reaction swath. This curve was determined several times with different annealing schemes, which ended up with the same result—the tunneling path is proceeding close to the MEP rather than to the LCT path. Along the optimal tunneling path, the ground-state tunneling splitting was calculated with a new semiclassical method introduced in an accompanying study [C. S. Tautermann, A. F. Voegele, T. Loerting, and K. R. Liedl, J. Chem. Phys. 117, 1967 (2002), following paper]. Another focus of investigation was the influence of deformation of the tunneling paths and a general scheme of determining an approximated optimal tunneling path at 0 K is introduced. © 2002 American Institute of Physics.

Notes: Charged systems interacting via Coulomb forces can be efficiently simulated by introducing a local, diffusing degree of freedom for the electric field. This paper formulates the continuum electrodynamic equations corresponding to the algorithm and studies the spectrum of fluctuations when these equations are coupled to mobile charges. I compare the calculations with simulations of a charged lattice gas, and study the dynamics of charge and density fluctuations. The algorithm can be understood as a realization of a mechanical model of the ether. © 2002 American Institute of Physics.

Notes: A new method for calculating the ground-state tunneling splitting is presented. It is based on the semiclassical theory including recently derived corrections and it is the first method, which explicitly takes into account the whole conformational space between the minima and the transition state. The density-functional theory is used to determine the qualitative shape of the potential energy surface (PES) and high level ab initio calculations provide information about the stationary points. With a dual level scheme, the low-level energy surface is mapped onto the high-level points to get a good quantitative description of the high-level PES. Therefore, the new method requires no adjustment of additional parameters like scaling of the energy barrier as is necessary in other methods. Once the high-level PES is calculated, the most probable tunneling paths are determined with a global optimization procedure. Along this representative tunneling path, the tunneling splitting is calculated with additional consideration of zero-point vibrational effects. The method is applied to three molecular systems, namely hydrofluoric acid dimer, malonaldehyde, and tropolone. These systems were chosen because their energy barriers differ strongly (1 kcal/mol–7 kcal/mol). The predicted tunneling splittings agree very well with the experimental ones, therefore, we expect our method to be generally applicable, independent of the magnitude of the energy barrier. © 2002 American Institute of Physics.

Notes: A simple relation between magnetic shielding and magnetizability has been found. Its validity has been first shown by numerical calculations and than proven analytically. This fundamental relation shows that the magnetic shielding tensor integrated over a whole space is simply proportional to the magnetizability tensor with a constant factor equal to two-thirds of the vacuum permeability with a negative sign. © 2002 American Institute of Physics.

Notes: We present an ab initio theoretical study of five low-energy isomers of the water hexamer {Chair, Cage(du)[1], Book, Prism, and Boat}, their intramolecular vibrations, binding energies De and dissociation energies D0. Møller–Plesset second order perturbation calculations using the aug-cc-pVTZ basis set at aug-cc-pVDZ optimized geometries including vibrational zero point energy corrections predict Chair to be the most stable isomer, followed closely by Cage(du)[1] (+0.02 kcal/mol) and Book (+0.05 kcal/mol), while Prism is 0.15 kcal/mol higher. The Boat conformer is least stable at both the De and D0 levels. The main focus is on the intramolecular normal modes of the five isomers. The calculated O–H stretching frequencies and intensities are compared to recent infrared spectra of water hexamer in supersonic jets, liquid-helium droplets and solid para-hydrogen matrices. The IR spectra indicate that Book and Chair are major species in the latter two environments and may also exist in supersonic jets. The (H2O)6 gas phase interconversion equilibria are calculated and predict that the most abundant isomer is Chair below 8 K, Cage between 8–26 K, and Book above 26 K. Several of the low-frequency vibrational modes are identified as low-amplitude precursors of the Chair↔Book↔Cage isomerization pathways. © 2002 American Institute of Physics.

Notes: The state-resolved and isotope-specific detection of nascent ClO generated from the photodissociation of Cl2O parent molecules is performed by observing single-color (2+1) resonance enhanced multiphoton ionization (REMPI) spectra following excitation in the wavelength range from 336 to 344 nm; additionally state-resolved detection of nascent ClO is performed by observing single-color two photon laser-induced fluorescence. The REMPI spectrum is assigned to the ClO(C 2Σ−,v′=0←X 2ΠΩ,v=0) transition. The population of rotational states up to J=130 is evidence of large rotational excitation induced by a strong dependence of the excited potential energy surface (PES) on the Jacobi angle γ. Formation of the 2Π3/2 spin-orbit state is preferred: P(2Π1/2):P(2Π3/2)=0.30±0.05 suggesting significant radiationless transfer from the excited PES to closely lying neighboring states. The anisotropy parameter for the transition is determined to be β=0.35 independent of the ClO quantum state. The principal excited electronic state is assigned to a 1B2←1A1 transition, in agreement with recent ab initio calculations. The measured β value is smaller than the limiting value of 1.0, suggesting dynamic reasons and simultaneous excitation to more than one PES. A fast and direct fragmentation process is deduced, where the upper limit for the lifetime of the excited dissociative state is calculated to be 300 fs. © 2002 American Institute of Physics.

Notes: We propose a detailed investigation of the schematic mode-coupling approach to experimental data, a method based on the use of simple mode-coupling equations to analyze the dynamics of supercooled liquids. Our aim here is to clarify different aspects of this approach that appeared so far uncontrolled or arbitrary, and to validate the results obtained from previous works. Analyzing the theoretical foundations of the approach, we first identify the parameters of the theory playing a key role and obtain simple requirements to be met by a schematic model for its use in this context. Then we compare the results obtained from the schematic analysis of a given set of experimental data with a variety of models and show that they are all perfectly consistent. A number of potential biases in the method are identified and ruled out by the choice of appropriate models. Finally, reference spectra computed from the mode-coupling theory for a model simple liquid are analyzed along the same lines as experimental data, allowing us to show that, despite the strong simplification in the description of the dynamics it involves, the method is free from spurious artifacts and provides accurate estimates of important parameters of the theory. The only exception is the exponent parameter, the evaluation of which is hindered, as for other methods, by corrections to the asymptotic laws of the theory present when the dynamics is known only in a limited time or frequency range. © 2002 American Institute of Physics.

Notes: Energy fluctuations of a solute molecule embedded in a polar solvent are investigated to depict the energy landscape for solvation dynamics. The system is modeled by a charged molecule surrounded by two layers of solvent dipolar molecules with simple rotational dynamics. Individual solvent molecules are treated as simple dipoles that can point toward or away from the central charge (Ising spins). Single-spin-flip Monte Carlo kinetics simulations are carried out in a two-dimensional lattice for different central charges, radii of outer shell, and temperatures. By analyzing the density of states as a function of energy and temperatures, we have determined the existence of multiple freezing transitions. Each of them can be associated with the freezing of a different layer of the solvent. © 2002 American Institute of Physics.

Notes: A generalized scattering matrix formalism is constructed to elucidate the interplay of electron resonance, coherence, dephasing, inelastic scattering, and heterogeneity, which play important roles in the physics of long-range electron transfer/transport. The theory consists of an extension of the standard Büttiker phase-breaking model and an analytical expression of the electron transmission coefficient for donor–bridge–acceptor systems with arbitrary length and sequence. The theory incorporates the following features: Dephasing-assisted off-resonance enhancement, inelasticity-induced turnover, resonance enhancement and its dephasing-induced suppression, dephasing-induced smooth superexchange-hopping transition, and heterogeneity effects. © 2002 American Institute of Physics.

Notes: The thermal effects on the ClCH2CN+Cl−SN2 reaction at 300 K have been studied by ab initio molecular dynamics. The role of the cyano-substituent is explained by the formation of a hydrogen bond and is discussed by comparison with the reactions of ClCH3 and Cl2CH2. It is shown that the interactions occurring in the prereactive complex are sufficiently strong to be effective also at high temperature. It is also shown that the temperature effects on the activation barrier are significant. © 2002 American Institute of Physics.

Notes: Proton transfer in ammonia–nitric acid clusters containing up to four component units are subject to theoretical calculation in this work. In a single ammonium nitrate unit, proton transfer between the nitric acid and ammonia unit does not occur but the two molecules are strongly hydrogen-bonded. In a cluster of two ammonium nitrate formula units [NH3HNO3]2, proton transfer does occur and the components are stabilized by ionic interactions. Ammonium nitrate solvated with single ammonia [NH3HNO3]NH3 or nitric acid [NH3HNO3]HNO3 molecules are also studied. Structural changes in the various clusters relative to the free molecules are discussed. Using population analysis, the total electrostatic interaction between the components of each cluster are calculated. It is argued that the magnitude of the total electrostatic interactions within the cluster determines whether proton transfer and ion formation takes place. Binding energies alone do not give a reliable indication of the occurrence of proton transfer. © 2002 American Institute of Physics.

Notes: The second dielectric virial coefficients of helium and argon are investigated using a fully quantum statistical approach and recent accurate ab initio results for the interatomic potentials and the interaction-induced polarizabilities. We thereby extend a preceding investigation based on a semiclassical approach to include quantum effects. For helium the results support the findings of a previous study by Moszynski et al. [J. Chem. Phys. 247, 440 (1995)] that quantum effects are substantial for temperatures below 10 K, while they are practically negligible above 70 K. For argon special care is needed in the numerical integrations carried out in the quantum statistical calculation of the virial coefficients, due to the presence of quasibound states in the continuum and a slow convergence of the summation over the angular momentum. Here quantum effects are practically negligible in the range of temperatures experimentally investigated, i.e., between 243 and 408 K. As far as comparison with experimental data is concerned, large discrepancies are found for some of the low-temperature experimental measurements of helium. Agreement is also unsatisfactory for high temperatures for argon and experimental redetermination is suggested. © 2002 American Institute of Physics.

Notes: The negative ion photoelectron spectra of the oxide anion complexes O−Rg, Rg=Ar, Kr, and Xe, and O−N2 have been recorded. In each spectrum, two partially resolved peaks were observed, their relative intensities varying with source conditions. These peaks were assigned to photodetachment transitions from the 2Σ ground state and unresolved 2Π3/2,1/2 low-lying excited states of the anion. From our data we find dissociation energies and bond lengths for the 2Σ and 2Π anion states. Periodic trends in the bond length and dissociation energy are examined and compared to those in the isoelectronic neutral halogen rare gas systems and the effect of anisotropy in the interatomic potential and relative interaction strength is examined. From our data we find that the dissociation energies in the anion system are much larger but that the 2Σ-2Π splitting is significantly lower. In addition to the diatomic clusters, we report the photoelectron spectra of the O−Krn=2–5 and O−Xen=2–3 clusters and tabulate the vertical detachment energies and peak widths. From a comparison of the energetics and peak broadening we are able to make a determination of the general structure of the n=2 and n=3 clusters. © 2002 American Institute of Physics.

Notes: The photoelectron spectra of the weakly bound KrO− anion are simulated using a theory which combines the atoms-in-molecule model for molecular electronic wave functions and the Rau–Fano model for photodetachment intensities [J. Chem. Phys. 112, 5852 (2000)]. The nonrelativistic potential energy curves of the anion are obtained from ab initio calculations. The calculated spectra and their temperature variation agree with the experimental data presented in the preceding article. The strong selectivity of the photodetachment process with respect to the symmetry of fine-structure components is rationalized and quantified. © 2002 American Institute of Physics.

Notes: Photodissociation spectra of free doubly charged anions IrX62− (X=Cl,Br) were measured in the photon energy range from 1.5 to 2.9 eV. Both data sets show the same features as the spectra of the respective aqueous solutions. Compared to solution, the gas phase absorption bands of IrBr62− are redshifted by 0.01–0.15 eV. For IrCl62− no such shift could be observed. Photodissociation of IrBr62− results in the formation of Br−, IrBr4−, and IrBr5−. Fluence dependent measurements of fragment formation as well as parent ion depletion, allowed inferences regarding the dissociation pathway and the inner barrier height for the dissociation process which was estimated to be 1.6±0.2 eV. From measurements of the kinetic energy released upon fragmentation into monoanions, we estimate the outer barrier height to be 2.2±0.2 eV. © 2002 American Institute of Physics.

Notes: Infrared spectra of the weakly bound complexes N2O–4He, N2O–3He, and OCS–3He have been observed using a tunable diode laser to probe a pulsed supersonic jet expansion. The rotational structure of the bands was analyzed using a conventional asymmetric rotor Hamiltonian. The N2O–3He and OCS–3He spectra are mostly a type (ΔKa=0) in structure, with very weak b-type (ΔKa=±1) transitions, but for N2O–4He the a- and b-type components are both prominent. The fitted rotational parameters are consistent with roughly T-shaped structures with intermolecular separations around 3.4–3.5 Å for N2O–He and 3.8–3.9 Å for OCS–He. The angle between the N2O or OCS axis and the He position is about 80° for N2O–He and 65° for OCS–He. The vibrational band origins are slightly blueshifted from those of the free molecule, with the N2O–He shifts (+0.2 cm−1) being about twice the magnitude of the OCS–He shifts (+0.1 cm−1). The results are of particular interest since N2O and (especially) OCS have both been used as probes in experiments on ultracold helium nanodroplets. © 2002 American Institute of Physics.

Notes: The multistate vibronic interactions in the benzene radical cation are investigated theoretically, employing the framework of a linear vibronic coupling scheme. The five lowest electronic states are included in the treatment; in view of the degeneracy of some states, this amounts to eight coupled potential energy surfaces. Different types of ab initio calculations have been performed for the system parameters and been found to be in good mutual agreement, thus supporting each other. The calculations reveal a whole sequence of low-energy conical intersections between the potential energy surfaces of different states. Their importance for the nuclear dynamics in this prototypical organic radical cation is pointed out. Wave-packet dynamical simulations for these coupled potential energy surfaces will be presented in the following paper (Paper II). © 2002 American Institute of Physics.

Notes: The multistate vibronic dynamics in the X˜ 2E1g-E˜ 2B2u electronic states of the benzene radical cation is investigated theoretically by an ab initio quantum-dynamical approach. The vibronic coupling scheme and the ab initio values of the system parameters are adopted from the previous Paper I. Vibronic line spectra are obtained with the Lanczos procedure. Extensive calculations on wave-packet propagation have been performed with the aid of the multiconfiguration time-dependent Hartree method. Up to five coupled electronic potential energy surfaces and 13 vibrational degrees of freedom have been included in these calculations. As a result, the impact of a third electronic state (X˜ or B˜) on a strongly coupled manifold (B˜-C˜ or D˜-E˜ states) is quantitatively assessed. It leads to a restructuring of the spectral envelope which is stronger for the B˜-D˜-E˜ than for the X˜-B˜-C˜ system. The internal conversion dynamics is characterized by a stepwise transfer of electronic population to the lowest electronic state on a time scale of ∼100 fs, if the system is prepared initially on the highest potential energy surface. Companion calculations have also been performed for the case when the system is prepared in the intermediate state at t=0; they show a branching of the electronic populations. These are all novel findings which are discussed in terms of a series of conical intersections between the various potential energy surfaces. The importance of such multistate vibronic interactions for the photophysics and photochemistry of medium-sized systems is pointed out. © 2002 American Institute of Physics.

Notes: A molecular theory of liquid phase vibrational energy relaxation (VER) [S. A. Adelman et al., Adv. Chem. Phys. 84, 73 (1993)] is applied to study the temperature T and density ρ dependencies of the VER rate constant k(T,ρ)=T1−1, where T1 is the energy relaxation time, of model Lennard-Jones systems that roughly simulate solutions of high-mass, low-frequency dihalogen solutes in rare gas solvents; specifically the I2/Xe, I2/Ar, and ICI/Xe solutions. For selected states of these systems, the theory's assumptions are tested against molecular dynamics (MD) results. The theory is based on the expression T1=β−1(ωl), where ωl and β(ω) are, respectively, the solute's liquid phase vibrational frequency and vibrational coordinate friction kernel. The friction kernel is evaluated as a cosine transform of the fluctuating force autocorrelation function of the solute vibrational coordinate, conditional that this coordinate is fixed at equilibrium. Additionally, the early-time decay of the force autocorrelation function is approximated by a Gaussian function which is exact to order t2. This Gaussian approximation permits evaluation of T1 in terms of integrals over equilibrium solute–solvent pair correlation functions. The pair correlation function formulas yield T1's in semiquantitative agreement with those found by MD evaluations of the Gaussian approximation, but with three orders of magnitude less computational effort. For the isothermal ρ dependencies of k(T,ρ), the theory predicts for all systems that the Gaussian decay time τ is nearly independent of ρ. This in turn implies that k(T,ρ) factorizes into a liquid phase structural contribution and a gas phase dynamical contribution, yielding a first-principles form for k(T,ρ) similar to that postulated by the isolated binary collision model. Also, the theory predicts both "classical" superlinear rate isotherms, and "nonclassical" sublinear isotherms similar to those recently observed by Troe and co-workers for azulene relaxation in supercritical fluids. The isochoric T dependencies of k(T,ρ) are studied in the range 300 to 1000 K. For none of the solutions are the rate isochores found to accurately conform to either Arrhenius or Landau–Teller kinetics. © 2002 American Institute of Physics.

Notes: The theoretical treatment in Paper I [D. W. Miller and S. A. Adelman, J. Chem. Phys. 117, 2672, (2002), preceding paper] of the vibrational energy relaxation (VER) of low-frequency, large mass dihalogen solutes is extended to the VER of the high-frequency, small mass molecular hydrogen solutes H2 and D2 in a Lennard-Jones argon-like solvent. As in Paper I, values of the relaxation times T1 predicted by the theory are tested against molecular dynamics (MD) results and are found to be of semiquantitative accuracy. To start, it is noted that standard Lennard-Jones site–site potentials derived from macroscopic data can be very inaccurate in the steep repulsive slope region crucial for T1. Thus, the H–Ar Lennard-Jones diameter σUV is not taken from literature values but rather is chosen as σUV=1.39 Å, the value needed to make the theory reproduce the experimental H2/Ar gas phase VER rate constant. Next, by MD simulation it is shown that the vibrational coordinate fluctuating force autocorrelation function 〈F˜(t)F˜〉0 of Paper I decays roughly an order of magnitude more rapidly for the molecular hydrogen solutions than for the dihalogen solutions. This result implies a relatively slow decay for the molecular hydrogen friction kernels β(ω)=(kBT)−1∫0∞〈F˜(t)F˜〉0 cos ω tdt, yielding for the H2/Ar and D2/Ar systems at T=150 K physical millisecond values for T1=β−1(ωl) despite the high liquid phase vibrational frequencies ωl of H2 and D2. The rapid decay of 〈F˜(t)F˜〉0 is due to both the steepness of the repulsive slope of the H–Ar potential and the small masses of H and D. Thus, the small value chosen for σUV is needed to avoid unphysically long T1's. Next, an analytical treatment of the H2/D2 isotope effect on T1, based on the theory, is found to predict that the H2/Ar and D2/Ar T1's are close in value due to the compensating effects of lower ωl but slower decay of 〈F˜(t)F˜〉0 for D2/Ar, a result in qualitative agreement with experiments. Applying the theory to numerically study the isothermal ρ dependencies of the VER rate constant k(T,ρ)=T1−1 at 150 K reveals that for both H2/Ar and D2/Ar, as for the solutions of Paper I, k(T,ρ) can be factorized as in the isolated binary collision (IBC) model. Moreover, the molecular theory and IBC rate isotherms differ only slightly for both solutions, a result interpreted in terms of the form of the H–Ar pair correlation function. The theoretical and experimental rate isotherms at 150 K are then compared. Agreement is very good for the H2/Ar solution, but for the D2/Ar solution the theoretical rates are about four times too large. Finally, the isochoric T dependencies of k(T,ρ) in the range 200–1000 K are found for both solutions to conform to an Arrhenius rate law. © 2002 American Institute of Physics.

Notes: Perovskites of the PbZr1−xTixO3 type are among the most important ferroelectric materials and highly active catalysts. The structural and electronic properties of PbTiO3, PbZrO3, and PbZr0.5Ti0.5O3 were examined using first-principles density-functional (DF) calculations with the local-density-approximation (LDA) or the generalized-gradient approximation (GGA, Perdew–Wang and Perdew–Burke–Ernzerhoff functionals). A series of crystal structures were considered for each compound. In several cases, the structural parameters predicted by the GGA functionals were clearly in better agreement with experimental results than the LDA-predicted values, but in qualitative terms the LDA and GGA approaches always predicted similar trends for crystal geometries and differences in thermochemical stability. DF calculations at the LDA level could underestimate the ferroelectric character of PbTiO3 and PbZr1−xTixO3. In the perovskites, the most stable structures belong to tetragonal (PbTiO3), orthorhombic (PbZrO3), and monoclinic (PbZr0.5Ti0.5O3) space groups. The positions of the Zr and Ti cations in the tetragonal and monoclinic phases of PbZr0.5Ti0.5O3 were determined. The calculated structural parameters give theoretical x-ray diffraction patterns that reproduce well experimental data. In general, Zr is much more rigid than Ti for displacements along the [001] direction in the cubic, tetragonal and monoclinic phases of PbZr1−xTixO3 compounds. The lead titanates/zirconates exhibit very strong metal↔oxygen↔metal interactions that drastically modify the electron density on the metal cations with respect to TiO2, ZrO2, and PbO. A similar phenomenon is observed in a series of ABO3 perovskites (A=Ca,Sr,Li,K,Na; B=Ti,Zr,Nb), and it is an important factor to consider when mixing AO and BO2 oxides for catalytic applications. © 2002 American Institute of Physics.

Notes: The photophysical properties of poly(2,5-pyridine diyl) (PPY) are characterized in detail using steady state measurements, time correlated single photon counting techniques, and excitation pulse energy dependence of the photoluminescence intensity. Our results show that the blue emission from dilute solution is prompt fluorescence (lifetime ∼10 ps). On the other hand, the green emission from thin solid film consists of phosphorescence as well as delayed fluorescence with lifetimes of ∼4.8 ns and 1.1 ns, respectively. Oxygen complexation plays an important role and increases triplet population in thin film. At low excitation pulse energies, emission in thin film originates mostly from the triplet state, while at higher excitation pulse energies, delayed fluorescence becomes the major part in the emission. Also, at low temperature (20 K), an excimerlike emission is observed to the red side of the PPY emission spectrum in thin film, which is only induced by pressure. © 2002 American Institute of Physics.

Notes: Using molecular dynamics simulations, we study dynamics of a model polymer melt composed of short chains with bead number N=10 in supercooled states. In quiescent conditions, the stress relaxation function G(t) is calculated, which exhibits a stretched exponential relaxation on the time scale of the α relaxation time τα and ultimately follows the Rouse dynamics characterized by the time τR∼N2τα. After application of shear γ(overdot), transient stress growth σxy(t)/γ(overdot) first obeys the linear growth ∫0tdt′G(t′) for strain less than 0.1 but saturates into a non-Newtonian viscosity for larger strain. In steady states, shear thinning and elongation of chains into ellipsoidal shapes take place for shear γ(overdot) larger than τR−1. In such strong shear, we find that the chains undergo random tumbling motion taking stretched and compact shapes alternatively. We examine the validity of the stress–optical relation between the anisotropic parts of the stress tensor and the dielectric tensor, which are violated in transient states due to the presence of a large glassy component of the stress. We furthermore introduce time-correlation functions in shear to calculate the shear-dependent relaxation times, τα(T,γ(overdot)) and τR(T,γ(overdot)), which decrease nonlinearly as functions of γ(overdot) in the shear-thinning regime. © 2002 American Institute of Physics.

Notes: Brownian dynamics simulations have been carried out to model attractive polymers in solution. Bead–spring polymer chains with attractions between the end beads were modeled over a wide concentration range on either side of the overlap concentration, ρ*, for the corresponding random coil polymers. The polymers were treated as beads linked by finitely extensible nonlinear elastic springs and the excluded volume repulsion between unlinked beads was represented by a pair potential with a Gaussian analytic form. For the sticky end-beads the potential has an attractive tail of Gaussian form. In addition to chains with purely repulsive bead–bead nonbonded interactions, three different systems with attactive end beads were modeled. There were those with (a) head–head (H–H) attractions, (b) with both H–H and tail–tail (T–T) attractions, and (c) with head–tail (H–T) attractions. The dimensions of the chains, the bead–bead radial distribution functions, as well as the dynamic properties such as stress tensor time-correlation functions, infinity frequency elastic modulus, and specific viscosity of the solution were calculated as functions of solution density and end-bead attraction class. We show that with the three classes of attractive end-bead functionality, the model polymers all depart from random coil statistics and show evidence of enhanced association, even in the dilute regime, especially for the H–T systems, which can form necklacelike structures at low dilution and micellelike states with increasing concentration. (Not all of the polymer statistics measures show major differences though.) However, only the rheology if the H–T system is markedly different from the random coil case. The rheology of the H–T system is quite different in qualitative and quantitative behavior to the other classes studied. There is a progressive retardation and increasingly near-exponential decay in the shear stress relaxation function. The viscosity of the H–T class of polymers is typically at least an order of magnitude higher than that of the others, even at concentrations far below the overlap concentration ρ* for such polymers. The infinite frequency elastic modulus is also typically about five times larger for the H–T class across the density range when compared with the other three types modeled. © 2002 American Institute of Physics.

Notes: The radial local dynamics in poly(1,4-trans-butadiene) melts is studied using molecular dynamics simulations. In this work, the hopping peak is observed in the Van Hove space-time correlation function Gs(r,t) for the first time in polymeric systems. The hopping motion, observed only for methine hydrogen, is also identified in the mean-square displacement 〈Δr2(t)〉 and the intermediate scattering function Fs(k,t) through the relative dynamics of the hopping methine hydrogen and the nonhopping methylene hydrogen. The hopping motion is found to cause an unusual broadening of the dispersion width in the dynamic structure factor Sinc(k,ω). Active free volume is proposed in terms of Gs(r,t) at a short time, which offers a consistency to the relationship between free volume and local dynamics. Fast counterrotation at a pair of CH(Single Bond)CH2 bonds across a CH(Double Bond)CH bond is found responsible for the hopping peak in this polymer, and a new hopping criterion modified for polymeric liquids is proposed. © 2002 American Institute of Physics.

Notes: A method for the calculation of the dielectric permittivity of isotropic and anisotropic homogeneous fluids is presented which, in the framework of the continuum approximation, adopts a realistic description of the molecular features, so overcoming some of the limits of the Onsager model. The Poisson equation for the molecular charge distribution contained in a cavity in a dielectric continuum in the presence of an external field is solved by a boundary element technique, which allows a detailed description of the cavity shape associated with a given molecular structure. The charge distribution is described in terms of point charges derived from ab initio calculations in vacuum, in addition to a set of interacting atom dipoles induced by all the electric fields experienced by the molecule in the condensed phase. The link between molecular features and bulk properties is established in a general way suitable for isotropic liquids and nematic phases, through the orientational distribution function of the molecule interacting with the applied field and the surrounding fluid. Numerical results are reported for the liquid phase of a set of selected organic compounds of different shape and polarity, and for the isotropic and nematic phases of 4-n-pentyl-4′-cyanobiphenyl (5CB). They show that a realistic description of the molecular features can have dramatic effects in the case of strongly anisometric molecules. © 2002 American Institute of Physics.

Notes: We investigate the Rubinstein–Duke model for polymer reptation by means of density-matrix renormalization group techniques both in the absence and presence of a driving field. In the former case the renewal time τ and the diffusion coefficient D are calculated for chains up to N=150 reptons and their scaling behavior in N is analyzed. Both quantities scale as powers of N:τ∼Nz and D∼1/Nx with the asymptotic exponents z=3 and x=2, in agreement with the reptation theory. For an intermediate range of lengths, however, the data are well fitted by some effective exponents whose values are quite sensitive to the dynamics of the end reptons. We find 2.7<z<3.3 and 1.8<x<2.1 for the range of parameters considered and we suggest how to influence the end reptons dynamics in order to bring out such a behavior. At finite and not too small driving field, we observe the onset of the so-called band inversion phenomenon according to which long polymers migrate faster than shorter ones as opposed to the small field dynamics. For chains in the range of 20 reptons we present detailed shapes of the reptating chain as function of the driving field and the end-repton dynamics. © 2002 American Institute of Physics.

Notes: We apply a Monte Carlo technique specialized for the simulation of rare events to study the activated counterions motions in the aluminosilicate Na+-mordenite. Mean activation barriers are obtained from minimum energy paths calculated on realistic potential energy surfaces by using a Metropolis algorithm. Energy barriers for Na+ hops calculated for lattices with various Si/Al ratio are found in good agreement with the Na+ detrapping energies measured by thermally stimulated current spectroscopy. One shows that the dielectric activated motions of Na+ proceed between degenerated many-body ground states with different dipolar moment by either sequential or collective hopping motions. This provides a first microscopic description of dielectric relaxation measured in zeolites. © 2002 American Institute of Physics.

Notes: A computationally efficient molecular dynamics implementation of a polarizable force field parametrized from ab initio data is presented. Our formulation, based on a second-order expansion of the energy density, models the density response using Gaussian basis functions derived from density functional linear response theory. Polarization effects are described by the time evolution of the basis function coefficients propagated via an extended Lagrangian formalism. We have devised a general protocol for the parametrization of the force field. We will show that a single parametrization of the model can describe the polarization effects of LiI in the condensed phase. © 2002 American Institute of Physics.

Notes: We show that the method of factorizing the evolution operator to fourth order with purely positive coefficients, in conjunction with Suzuki's method of implementing time-ordering of operators, produces a new class of powerful algorithms for solving the Schrödinger equation with time-dependent potentials. When applied to the Walker–Preston model of a diatomic molecule in a strong laser field, these algorithms can have fourth order error coefficients that are three orders of magnitude smaller than the Forest–Ruth algorithm using the same number of fast Fourier transforms. Compared to the second order split-operator method, some of these algorithms can achieve comparable convergent accuracy at step sizes 50 times as large. Morever, we show that these algorithms belong to a one-parameter family of algorithms, and that the parameter can be further optimized for specific applications. © 2002 American Institute of Physics.

Notes: The most common form of density functional calculations on molecular systems used generalized gradient approximation exchange-correlation functionals (such calculations can be applied to larger systems because no exact exchange is included). The most efficient and fastest such codes use an auxiliary basis set to fit the density so that only three-center integrals need to be evaluated. The codes DGAUSS and TURBOMOL use Gaussian basis sets, whereas the long-established ADF code uses Slater basis sets. We here examine the use of Slater basis sets. Our new code evaluates all required integrals numerically by quadrature. We report calculations on the G2 molecular set, contrasting them with similar calculations using Gaussian basis sets. Our conclusion, as far as energetics and structure are concerned, is that very similar predictions may be obtained from basis sets of the same size, and at approximately the same cost. © 2002 American Institute of Physics.

Notes: The ground states of the positronic complexes LiPs, NaPs, e+Be, e+Mg, and of the parent ordinary-matter systems have been simulated by means of the all-electron fixed-node diffusion Monte Carlo (DMC) method. Positron affinities and positronium binding energies are computed by direct difference between the DMC energy results. LiPs was recomputed in order to test the possibility of approximating the electron–positron Coulomb potential with a model one that does not diverge for r=0, finding accurate agreement with previous DMC results. As to e+Be, the effect due to the near degeneracy of the 1s22s2 and 1s22p2 configurations in Be is found to be relevant also for the positron affinity, and is discussed on the basis of the change in the ionization potential and the dipole polarizability. The DMC estimate of the positron affinity of Mg, a quantity still under debate, is 0.0168(14) hartree, in close agreement with the value 0.015 612 hartree computed by Mitroy and Ryzhihk [J. Phys. B. 34, 2001 (2001)] using explicitly correlated Gaussians. © 2002 American Institute of Physics.

Notes: Liquid-state integral equation methods are employed to study the thermodynamic and structural properties of ideal and repelling rigid rods mixed with hard spheres in the limits when one of the species is dilute. The role of rod aspect ratio and sphere/rod size asymmetry is explored over a wide range of system parameters encompassing the colloid, nanoparticle, and crossover regimes. Novel predictions are found for the polymer (sphere) mediated depletion potentials and second virial coefficients of particles (rods) in dense polymer (sphere) suspensions. The adequacy of the closure approximations employed is tested by comparison with available numerical calculations and more rigorous theories in special limits. The liquid-state theory appears to be accurate for all properties in the nanoparticle regime and for the insertion chemical potential of needles and spherocylinders. However, it significantly underestimates depletion attractions effects in the colloidal regime of short rods and large spheres due to nonlocal entropic repulsion effects between polymers and particles not captured by the classic Percus–Yevick approximation. © 2002 American Institute of Physics.

Notes: A parallel searching algorithm based on eigenvector-following is used to generate databases of minima and transition states for all-atom models of Ac(ala)12NHMe and Ac(ala)16NHMe. The AMBER95 force field of Cornell et al. [J. Am. Chem. Soc. 117, 5179 (1995)] is employed both with and without a simple implicit solvent. We use a master equation approach to analyze the dynamics of both systems, and relate the results to the potential energy landscapes using disconnectivity graphs. The low-lying regions of both energy landscapes are compared and found to be remarkably similar. α-helix formation occurs via an ensemble of pathways involving both the N- and C-termini. The global minima of the two systems are also located using the CHARMM22 force field of Mackerell et al. [J. Phys. Chem. B 102, 3586 (1998)], for comparison with AMBER95. © 2002 American Institute of Physics.

Notes: Second moments (M2) and spin-lattice relaxation times (T1) of proton nuclear magnetic resonance (NMR) measurements were analyzed for the hydrogensulfides of sodium, potassium, and rubidium. Three modifications are known at ambient pressure for each of MHS with M=Na, K,Rb. They are isotypic for the different cations, with an ordered monoclinic low temperature modification, a dynamically disordered rhombohedral middle temperature modification (MTM), and a cubic high temperature modification (HTM). The number of proton sites for one proton increases from two in the MTM to at least eight in the HTM. The title compounds were investigated by proton NMR in the temperature range 180 K≤T≤560 K with a resonance frequency of ν0=400 MHz. In addition, KHS was measured from T=90 K up to T=297 K, using proton resonance frequencies ν0=45 MHz and ν0=96 MHz. The M2 calculated from the absorption signal of the individual modification for the various cations do not differ from those calculated for the known crystal structures. The minima of T1 are in good agreement with results of calculations based on the crystal structure of the MTM of these compounds and two-site 180° reorientations of the anions. Activation enthalpies and attempt frequencies (NaHS:26.2(4) kJ mol−1, 6.1(2)×1014 s−1, KHS:19.0(4) kJ mol−1, 4.0(7)×1013 s−1, RbHS:16.3(4) kJ mol−1, 2.0(2)×1013 s−1) agree with those determined by quasielastic neutron scattering [Haarmann et al., J. Chem. Phys. 113, 8161 (2000)]. For the cubic HTM, translational motion of the cations is probable. Only in the case of KHS and RbHS, was the onset of this translational motion already observed in the MTM at temperatures close to the phase transition into the HTM. This seems to be a difference in the mechanism of the MTM(arrow-right-and-left)HTM phase transition for NaHS, on the one hand, and KHS, RbHS, on the other hand. © 2002 American Institute of Physics.

Notes: We report a Brownian dynamics (BD) simulation study of the Förster energy transfer in a dye-labeled Rouse polymer chain. The simulation method is based on the normal mode BD propagation and numerical path integration of the survival probability. It is shown that a properly constructed truncated normal-mode approximation (TNMA) can speed up the simulations considerably, without essential loss of accuracy. In particular, an effective-sink TNMA scheme is found to be quite efficient. The idea is based on a standard time scale separation ansatz, where all the normal modes are separated into slow and fast, in terms of the corresponding relaxation times. The fast normal modes are assumed to be equilibrated in the course of reaction and thus can be integrated out. Their effect is to modify the reaction sink for the slow modes. The first-order approximation can be handled most easily, without a simulation. Even this simple approximation can be preferable to the well-known Wilemski–Fixman approximation, if the reaction sink is wide, i.e., when the Förster radius exceeds the polymer mean bond length, the condition often chosen in experiments on polymer folding. © 2002 American Institute of Physics.

Notes: The effects of confining a polyelectrolyte solution containing a polyion and its counterions in spherical cavities of different sizes have been investigated by Monte Carlo simulations using a simple model system. Polyions of three different linear charge densities and counterions of three different valences have been examined. Structure, energy, and free energy properties of all systems have been determined as a function of the sphere radius. In nearly all cases, the free energy of confining the polyelectrolyte solution increases as the sphere radius is decreased. The free energy cost decreases as the linear charge density of the polyion is reduced and as the counterion valence is increased, although for completely different reasons. A strong consistency among the variations of the different structural and thermodynamic results on the linear charge density and counterion valence was found. The study provides information relevant for experimental systems of polyions confined to, e.g., viruses, vesicles, and zeolite cavities. © 2002 American Institute of Physics.

Notes: Electrochemical electron-transfer reactions form the basis of such important devices as fuel cells and sensors. Previous theories of these reactions were limited either to the case of weak electronic interaction between the electrode and the reactants, or to strong interactions. In this work the rate of electron exchange is calculated by a combination of quantum mechanics and computer simulations. This method is valid for all strengths of the electronic interaction, so that the dependence of the reaction rate on the interaction strength could be obtained. Our results encompass three different regimes; in the order of increasing interaction these are: (i) a linear region, in which first-order perturbation theory holds; (ii) a weakly adiabatic region, in which the rate is limited by solvent dynamics; (iii) a strongly adiabatic region, in which the interaction lowers the energy of activation. © 2002 American Institute of Physics.