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 mos t molecules of interest, the spectral information is either not comple te or available. For those few where they are available, simple freque ncy information is not sufficient to generate the constraints for a fo rce field (FF) derivation. Full normal mode information can only be ob tained by extensive isotopic substitution data. On the other hand, pra ctical levers of ab initio quantum chemical (QC) calculations usually leads to errors in vibrational frequencies that are too large. We deve loped a method to combine available experimental frequencies with ab i nitio normal mode calculations to generate the Hessian biased force fi eld (HBFF) which reproduces molecular Vibrations at level of accuracy sufficient for spectroscopy. Combined with other data, these QC calcul ations also determine accurate values for other FF parameters (charges , torsion potentials and hydrogen bond potentials) needed in a complet e FF. Aspects of each type of force constants derived using HBFF metho dology is illustrated with a variety of calculations on small molecule s. These FF are then applied to larger molecules with similar chemical identity, e.g. nylons from N-methylacetamide. (C) 1997 Published by E lsevier Science B.V.