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  • 1
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 117 (2002), S. 1450-1456 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    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.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 116 (2002), S. 2852-2862 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: An accurate method to compute the annihilation rate in positronic systems by means of quantum Monte Carlo simulations is tested and compared with previously proposed methods using simple model systems. This method can be applied within all the quantum Monte Carlo techniques, just requiring the accumulation of the positron–electron distribution function. The annihilation rate of e+LiH as a function of the internuclear distance is studied using a model potential approach to eliminate the core electrons of Li, and explicitly correlated wave functions to deal with all the remaining particles. These results allow us to compute vibrationally averaged annihilation rates, and to understand the effect of the Li+ electric field on positron and electron distributions. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 69-76 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We report ground state energies and structural properties for small helium clusters (4He) containing a H− impurity computed by means of variational and diffusion Monte Carlo methods. Except for 4He2H− that has a noticeable contribution from collinear geometries where the H− impurity lies between the two 4He atoms, our results show that 4HeNH− clusters have a compact 4HeN subsystem that binds the H− impurity on its surface. The results for N≥3 can be interpreted invoking the different features of the minima of the He–He and He–H− interaction potentials. © 2000 American Institute of Physics.
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  • 4
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 6755-6758 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Very accurate energies can be computed by the fixed-node diffusion Monte Carlo method. They are affected only by the nodal error due to the approximate description of the nodal surfaces by the trial wave function. We examine the cancellation of nodal errors in molecular electron affinity calculations. Ground state energies of the anions of first-row hydrides AH (A=Li–O) have been computed using the fixed-node diffusion Monte Carlo method with a determinant times a correlation factor as the trial wave function. The energies are among the lowest to date. Using the energy values for the neutral molecules computed by Luchow and Anderson [A. Luchow and J. B. Anderson, J. Chem. Phys. 105, 7573 (1996)] we computed adiabatic electron affinity values and found them in agreement with the experimental data. As a consequence, the values of the anion dissociation energies are also correctly evaluated. © 1999 American Institute of Physics.
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  • 5
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 114 (2001), S. 10579-10582 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The energies at geometries close to the equilibrium for the e+LiF and e+BeO ground states were computed by means of diffusion Monte Carlo simulations. These results allow us to predict the equilibrium geometries and the vibrational frequencies for these exotic systems, and to discuss their stability with respect to the various dissociation channels. Since the adiabatic positron affinities were found to be smaller than the dissociation energies for both complexes, we propose these two molecules as possible candidates in the challenge to produce and detect stable positron–molecule systems. Moreover, low-energy positron scattering on LiF and BeO targets may show vibrational Feshbach resonances as fingerprints of the existence of stable ground states of e+LiF and e+BeO. © 2001 American Institute of Physics.
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  • 6
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 6154-6159 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The potential energy curve of e+LiH has been computed by means of diffusion Monte Carlo using explicitly correlated trial wave functions. This curve allows us to compute the adiabatic total and binding energies and the vibrational spectrum of e+LiH, and the adiabatic positron affinity of LiH. Using these results, we discuss the possibility to detect spectroscopically e+LiH in the gas phase, in order to have the first direct observation of a positron-containing system. © 2000 American Institute of Physics.
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  • 7
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We discuss the accuracy and applicability to molecular systems of the model proposed by G. G. Ryzhikh and J. Mitroy [J. Phys. B 31, 5013 (1998), ibid. 32, 2203 (1999)] to compute annihilation rate in positron-containing systems. Moreover, we propose small modifications to this model in order to improve its accuracy for molecular systems. © 2000 American Institute of Physics.
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  • 8
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 1063-1065 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The interaction potential between H and PsH, and between H and H− have been calculated using the diffusion Monte Carlo method. As far as the H–H− system is concerned, our results appear to be the most accurate published so far. The potential functions show the H–PsH system to be bound with respect to the H2+Ps dissociation channel for nuclear distances larger than 3.5 bohr, while the H2−−e+ threshold lies 0.25 hartree above for all the distances studied. Moreover, the H–PsH potential curve presents a minimum located at 5.2 bohr that can be attributed to the van der Waals interaction between the two fragments. The relevancy of these results for the low energy scattering of Ps on H2 and for the exchange reaction of H with Ps is discussed. © 2000 American Institute of Physics.
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  • 9
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 108-114 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Many different properties of the positron containing systems PsH, [Li,e+], LiPs, and [LiH,e+], were computed using both variational Monte Carlo and fixed node diffusion Monte Carlo methods, and explicitly correlated trial wave functions. Our results show that these techniques can accurately compute not only energy values, but also other observables. Our 〈δ(r+−)〉 values for PsH, [Li,e+], and LiPs are in good agreement with the most recent state of the art correlated calculations, while for [LiH,e+] our calculations are the first to give reliable results. © 1999 American Institute of Physics.
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