EXCITED POTENTIAL-ENERGY SURFACES OF CH3SH FROM THE AB-INITIO EFFECTIVE VALENCE SHELL HAMILTONIAN METHOD

The ground and first and second (1)A'' potential surfaces of methyl me rcaptan (CH3SH) are calculated as a function of the C-S and S-H bond c oordinates using the ab initio effective valence shell Hamiltonian (H( n)u) method. The computations for this highly nontrivial system provid e the first serious tests for choosing restricted valence spaces and f or computing global potential energy surfaces with the H(n)u methods. The quasidegeneracy constraints on the H(n)u method suggest choosing a valence space which consists of the two (3a'' and 10a') highest energ y occupied orbitals in the ground state and the three lowest a' unoccu pied orbitals. The global potential surfaces are computed with a modes t basis, but larger basis set computations at selected geometries test convergence for vertical excitation energies, ionization potentials, and C-S and S-H bond energies. The calculations are compared to both e xperiment and other calculations for this system. The computations ass ist in the interpretation of CH3SH photodissociation dynamics observed by Butler and co-workers.