The Potential Energy Surface of Hydrogen Sulfide

We report here a refinement of the potential energy surface for the electronic ground state of the H2S molecule in a least-squares fit to experimental spectroscopic data. The calculations are carried out by combining an exact kinetic energy (EKE) approach to the calculation of the rotation–vibration energies of a triatomic molecule with the MORBID approach, which employs an approximate kinetic energy operator. The combined method has been described and applied to the water molecule in two previous publications [O. L. Polyansky, P. Jensen, and J. Tennyson, J. Chem. Phys. 101, 7651–7657 (1994); ibid., submitted for publication.] . The input data for the fit of the present work comprised rotation– vibration term values with J £ 10 for the vibrational ground state and the n2 state, and with J £ 5 for all other states that have been characterized by high-resolution spectroscopy. In total, we fit data for 31 vibrational states of H2S. We have obtained a very accurate potential within the framework of the EKE approach. The root-mean-square deviation for the fitted vibrational energies was found to be 0.28 cm . The fit to all 450 input data had a standard deviation of 0.17 cm . In the final fit, 11 parameters were varied. Our analysis of the residuals (observed 0 calculated) from the fit provides even stronger evidence 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, the J , Ka-dependent nonadiabatic corrections to the kinetic energy operator for the nuclear motion must be explicitly considered. q 1996 Academic Press, Inc.