Potential energy surface and vibrational spectrum of OCS

A potential energy surface for the electronic ground state of OCS is optimized by means of the self-consistent field - configuration interaction method using the exact vibrational Hamiltonian in the bond length-bond angle coordinates. In the optimization, the LMF optimizer in the non - linear least - squares refinement is employed to fit the recently observed band origins below 8000 cm(-1). The standard deviation of the final fitting for the 50 vibrational levels is about 0.08 cm(-1) with the largest deviation of - 0.37 cm(-1). The vibrational energy levels for some isotopomers of OCS are also investigated using the optimized potential and in excellent agreement with the observed values.