Investigation of Electron Correlation-Effects on Molecular Geometries

by Teppen, B. J.; Miller, D. M.; Cao, M.; Frey, R. F.; Newton, S. Q.; Momany, F. A.; Ramek, M.; Schafer, L.

A survey is presented as part of an ongoing program in which the effects of electron correlation on molecular structures are studied for the purpose of obtaining data that can be used in developing force field parameters for empirical molecular modeling. In our approach, molecular structures devoid of dispersion forces, i.e. optimized at the Hartree-Fock (HF) level, are compared with the same structures optimized at the MP2 level, in which the dispersion forces are ''switched on''. So far, the investigations have included hydrocarbons, to study aliphatic non-bonded interactions, systems with internal hydrogen bonding (ethylene glycol, glycerol, glycine and the model dipeptide N-formyl alanine amide), and silicates, which are studied in order to derive force field parameters applicable to modeling clays. In hydrocarbons, electron correlation effects lead to contractions in 1,5-non-bonded distances in all rotamers with gauche (G) torsions, particularly in GG sequences, in which small changes in torsional angles (< 5-degrees) can cause large changes (a few tenths of an angstrom) in non-bonded distances in the attractive region of the van der Waals potential. In ethylene glycol and glycerol, hydrogen bonds in MP2 structures are 0.1 to 0.2 A shorter than the same bonds in the corresponding HF structures. In silicates, significant correlation effects are found both in primary structural parameters and torsional angles, and again long-range non-bonded distances are contracted in structures in which correlation effects are active. In the context of the widely used single-point energy calculations at the correlation level (for example single-point MP2 energies calculated for HF geometries), we find that the errors connected with the lack of geometry optimization at the MP2 level increase with the size of a system and are larger for folded conformations than stretched ones.

Journal of Molecular Structure-Theochem
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