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.

Key words: POTENTIAL ENERGY SURFACES, EXCITED STATE, OCS