Ab initio derived spectroscopic quality force fields for molecular modeling and dynamics

Vibrational frequencies of a molecule are the most direct experimental information about force constants for that molecule. However, for most molecules of interest, the spectral information is either not complete or available. For those few where they are available, simple frequency information is not sufficient to generate the constraints for a force field (FF) derivation. Full normal mode information can only be obtained by extensive isotopic substitution data. On the other hand, practical levels of ab initio quantum chemical (QC) calculations usually leads to errors in vibrational frequencies that are too large. We developed a method to combine available experimental frequencies with ab initio normal mode calculations to generate the Hessian biased force field (HBFF) which reproduces molecular vibrations at level of accuracy sufficient for spectroscopy. Combined with other data, these QC calculations also determine accurate values for other FF parameters (charges, torsion potentials and hydrogen bond potentials) needed in a complete FF. Aspects of each type of force constants derived using HBFF methodology is illustrated with a variety of calculations on small molecules. These FF are then applied to larger molecules with similar chemical identity, e.g. nylons from N-methylacetamide.