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The potential energy surface of the glycine dimer was investigated by the molecular dynamics/quenching method. A new empirical potential (EP1), largely based on the standard AMBER force field of Cornell et al. (Cornell, W. D.; Cieplak, P.; Bayly, C. I.; Gould, I. R.; Merz, K. M.; Ferguson, D. M:; Spellmeyer, D. C.; Fox, T.; Caldwell, J. E.; Kollman, P. J. Am. Chem. Soc. 1995, 117, 5179), was introduced. It employs atomic polarizabilities and RESP/B3LYP/aug-cc-pVTZ atomic charges and well mimics the high-level ab initio PES of the glycine dimer. Surprisingly, the most stable structure of the glycine dimer determined with the EP1 potential does not correspond to the planar cyclic structure with two O-H...O hydrogen bonds (C1) but to a stacked arrangement (S1). The stabilization energies of the 22 lowest-energy isomers of the glycine dimer were recalculated at the MP2/6-31G** level of theory, and the largest difference was found for the C1 and S1 structures. While the empirical potential favored the S1 structure by 3 kcal/mol, the correlated ab initio MP2/6-31G** calculations preferred the C1 structure by 2 kcal/mol. Applying higher levels of ab initio calculations (countrepoise corrected gradient optimization, larger basis sets, CCSD, CCSD(T), and QCISD(T) methods), we found that both structures are comparable in energy or that the stacked structure is even slightly more stable. The finding that the cyclic C1 structure is not the lowest energy configuration is in good agreement with IR spectroscopic studies on glycine dimers trapped in liquid helium droplets. All experimental spectra feature a strong absorption band in the region of the free O-H stretch, indicating that both hydroxyl groups cannot be hydrogen-bonded in the glycine dimer at the same time. Due to fast (subnanosecond) cooling of the glycine dimer in the helium droplets, it is also suggested that a T-shaped local minimum can be preserved in the cluster.
