TRANSFER OF MOLECULAR PROPERTY TENSORS IN CARTESIAN COORDINATES - A NEW ALGORITHM FOR SIMULATION OF VIBRATIONAL-SPECTRA

A direct transfer of Cartesian molecular force fields (FF) and electri c property tensors is tested on model systems and compared to transfer in internal coordinates with an aim to improve simulation of vibratio nal spectra for larger molecules. This Cartesian transformation can be implemented easily and offers greater flexibility in practical comput ations. It can be also applied for transfer of anharmonic derivatives. The results for model calculations of the force field and vibrational frequencies for N-methylacetamide show that our method removes errors associated with numerical artifacts caused by nonlinearity of the oth erwise required Cartesian to internal coordinate transformation. For d etermination of IR absorption and vibrational circular dichroism inten sities, atomic polar and axial tensors were also transferred in the Ca rtesian representation. For the latter, which are dependent upon the m agnetic dipole operator, a distributed origin gauge is used to avoid a n origin dependence. Comparison of the results of transferring ab init io FF and intensity parameters from an amide dimer fragment onto a tri peptide with those from a conventionally determined tripeptide FF docu ment some limitations of the transfer method and its possible applicat ions in the vibrational spectroscopy. Finally, application to determin ation of the FF and spectra for helical heptapeptide are presented and compared to experimental results. (C) 1997 by John Wiley & Sons, Inc.