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  • 1
    ISSN: 1520-6904
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    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 107 (1997), S. 6770-6781 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The conformational equilibrium of gaseous glycine presents a severe challenge to quantum chemistry and, in particular, to density functional theory (DFT), due to the presence of internal hydrogen bonds. We present new DFT results for the structure and the energetics of glycine and malonaldehyde, using the recently developed nonlocal exchange-correlation functionals BLap1 and PLap1. A comparative analysis is made with the results of the generalized-gradient-approximation (GGA) schemes Becke–Perdew (BP86) and Becke–Lee–Yang–Parr (BLYP), the hybrid Hartree–Fock (HF)-DFT methods B3PW91 and B3LYP, with post-HF methods, and with the available experimental data. Our BLap1/TZVP and PLap1/TZVP values for the energy margin between the two lowest conformers of glycine (0.84 and 1.05 kcal/mol, respectively) are within the experimental error bars (1.0±0.5 kcal/mol). MP2 underestimates this energy difference by about 0.5 kcal/mol, BLYP and the hybrid methods are off by about 0.9 kcal/mol. The optimized geometry of malonaldehyde is improved with the Lap1 functionals, compared to the GGA results reported previously. Improvement over the GGA is also achieved for the energy barrier for the internal proton transfer in malonaldehyde. Recent high quality post-HF (G2) calculations of Barone and Adamo1 yield 4.4 kcal/mol. The best GGA (PP86) value of 2.1 kcal/mol is seriously underestimated. The best B3LYP estimate is 3.0 kcal/mol.1 Our BLap1/TZVP (3.28 kcal/mol) and PLap1/TZVP (4.56 kcal/mol) values are in the correct energy range, the latter being, in fact, very close to the G2 estimate. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 0192-8651
    Keywords: glycine ; conformational equilibrium ; density functional theory ; biomolecular modeling ; microwave structure ; adiabatic connection method ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Computer Science
    Notes: The potential energy surface of un-ionized glycine has been explored with density functional theory. The performance of several nonlocal functionals has been evaluated and the results are presented in the context of available experimental information and post-Hartree-Fock quantum chemical results. The zero-point and thermal vibrational energies along with vibrational entropies play a very important role in determining the relative stability of glycine conformers; the realization of this has led to some revision and reinterpretation of the experimental results. Uncertainties in the vibrational contributions to the energy differences of several tenths of a kilocalorie/mole remain. The uncertainty in the vibrational free energy is even larger, about 1 kcal/mol. In the final analysis, we suggest that the best estimate of the electronic energy difference between the two lowest glycine conformers should be revised downward from 1.4 to 1.0 kcal/mol. Thirteen stationary points on the potential energy surface have been localized. For the majority of these, there is close agreement among various nonlocal density functionals and the post-Hartree-Fock methods. However, the second conformer (IIn), which has a strong hydrogen bond between the hydroxyl hydrogen and the nitrogen of the amine group, presents a distinct challenge. The relative energy of this conformer is extremely sensitive to the basis set, the level of correlation, or the functional used. The widely used BP86, PP86, and BP91 nonlocal functionals overestimate the strength of the hydrogen bond and predict that this conformer is the lowest energy structure. This contradicts both experiment and high-level post-Hartree-Fock studies. The adiabatic connection method (ACM) and the BLYP functional yield the correct order. The ACM method, in particular, gives energies which are in reasonable agreement with MP2, although these are somewhat low as compared with experiment. Based on this study, ACM should perform well for this type of bioorganic application, with typical errors of a few tenths of a kilocalorie/mole and only rarely exceeding 0.5 kcal/mol.   © 1997 John Wiley & Sons, Inc.   J Comput Chem 18: 1609-1631, 1997
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The kinetic energy density-dependent correlation functional LAP1 is extended to include parallel-spin correlation beyond the exchange level. Two exchange-correlation schemes are considered, combining the new correlation functional (LAP3) with the GGA exchange of Becke and the GGA exchange of Perdew. Extensive tests on molecules and hydrogen-bonded systems are presented and discussed elucidating the role of parallel-spin correlation in different cases. Its inclusion in the LAP functional leads, on average, to a slight improvement of the calculated binding energies and equilibrium geometries of molecules. Particularly high sensitivity of the energy results on the relative share of parallel-spin correlation is observed for aromatic molecules and for systems involving weak hydrogen bonds.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 64: 427-446, 1997
    Additional Material: 11 Tab.
    Type of Medium: Electronic Resource
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