Excited vibrational states and potential energy function for OCS determined using generalized internal coordinates

Variational calculations of excited vibrational states for the OCS molecule , using generalized internal coordinates properly optimized, are presented. The calculations are made for two empirical and one ab initio potential en ergy surfaces previously reported. It is shown that the computed vibrationa l frequencies differ considerably from the experimental values for the thre e potential surfaces employed. Consequently a new and much more accurate po tential surface is determined for OCS by nonlinear least-squares fitting to the observed vibrational terms. The surface is expressed as a Morse-cosine expansion in valence coordinates and its quality is checked by computing t he vibrational frequencies of three isotopic species of the molecule. (C) 2 000 American Institute of Physics. [S0021- 9606(00)00738-8].