THE POTENTIAL-ENERGY SURFACE OF HYDROGEN-SULFIDE

We report here a refinement of the potential energy surface for the el ectronic ground state of the (H2S)-S-32 molecule in a least-squares fi t to experimental spectroscopic data. The calculations are carried out by combining an exact kinetic energy (EKE) approach to the calculatio n of the rotation-vibration energies of a triatomic molecule with the MORBID approach, which employs an approximate kinetic energy operator. The combined methods has been described and applied to the water mole cule in two previous publications [O.L. Polyansky, P. Jensen, and J. T ennyson, J. Chem. Phys. 101, 7651-7657 (1994); ibid, submitted for pub lication.]. The input data for the fit of the present work comprised r otation-vibration term values with J less than or equal to 10 for the vibrational ground state and the nu(2) state, and with J less than or equal to 5 for all other states that have obtained a very accurate pot ential within the framework of the EKE approach. The root-mean-square deviation for the fitted vibrational energies was found to be 0.28 cm( -1). The fit to all 450 input data had a standard deviation of 0.17 cm (-1). In the final fit, 11 parameters were varied. Our analysis of the residuals (observed-calculated) from the fit provides even stronger e vidence than analogous fits for water that we have reached a level of accuracy at which the breakdown of the Born-Oppenheimer approximation becomes the most important reason for the deviation between theory and experiment. In order to improve further the fit, J, K-d-dependent non adiabatic corrections to the kinetic energy operator for the nuclear m otion must be explicitly considered. (C) 1996 Academic Press, Inc.