Calculations on species relevant to the photoionization of the van der Waals molecule (H2S)2

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Abstract
Ab initio geometry optimizations are performed on the neutral and cation of H2S and (H2S)2. The neutral dimer is a weakly bound complex possessing only shallow minima in the potential energy surface, but the dimer cation has a deeper minimum where a proton has been transferred to give a complex of H3S+ and HS. Dimer binding energies, ionization energies, and appearance potentials are calculated. Intermonomer distances can be quite sensitive to basis set and correlation improvements on the HF/6‐31G* level of theory. The potential energy surfaces, however, are flat enough so that the energetic consequences are not large. A simple procedure for estimating energy changes involving ionization is suggested; this method separates an adiabatic energy difference into a vertical ionization followed by a geometric relaxation.