— —   A Density Functional Study of the Glycine Molecule: Comparison with Post-Hartree  Fock Calculations and Experiment D. T. NGUYEN, 1 A. C. SCHEINER, 1 J. W. ANDZELM, 1 S. SIROIS, 2 * D. R. SALAHUB, 2 A. T. HAGLER 1† 1 Biosym Technologies, Inc., ‡ 9685 Scranton Road, San Diego, California 92121 2 Departement de Chimie, Universite de Montreal, CP 6128 Succursale Centreville, Montreal, Quebec ´ ´ ´ ´ ´ H3C 3J7, Canada Received 14 November 1996; accepted 26 March 1997 ABSTRACT: 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- Ž . Hartree  Fock methods. However, the second conformer IIn , which has a strong hydrogen bond between the hydroxyl hydrogen and the nitrogen of the * Also affiliated with the Center for Research on Computa- Ž . tion and Its Applications CERCA , Montreal, Quebec, Canada ´ ´ † Present address: Science Media, 6540 Lusk Blvd., Suite C144, San Diego, CA 92121. ‡ Now Molecular Simulations, Inc. Correspondence to: A. T. Hagler Contractgrant sponsors: National Institute of Technologies; NSERC; FCAR; CERCA ( ) Journal of Computational Chemistry, Vol. 18, No. 13, 1609 1631 1997  1997 John Wiley & Sons, Inc. CCC 0192-8651 / 97 / 131609-23