Optimal generalized internal vibrational coordinates and potential energy surface for the ground electronic state of SO2

An optimization of generalized internal vibrational coordinates for the electronic ground state of the SO2 molecule is carried out. These coordinates are given by the magnitudes and the angle formed by two vectors expressed as linear combinations of the internal valence vectors, and depend on two external parameters which can be optimized so as to reduce the vibrational coupling of the molecule as much as possible. The optimal values of the parameters are found by minimizing a set of unconverged vibrational energies which are computed variationally using a small basis function set. It is shown that the optimal internal coordinates obtained for SO2 are superior to both valence and Radau coordinates, as well as to a set of normal coordinates previously derived by proper rotation of the Jacobi S-O-2 coordinates. These optimal internal coordinates are then applied to calculate the vibrational energies of the molecule using an ab initio force field expressed as a Morse-cosine expansion, and then to refine it by nonlinear least-squares fitting to the observed vibrational frequencies. (C) 2001 American Institute of Physics.