Ab initio potentials for (H₂S)₂, including studies of effects from third-order many-body perturbation theory and three-body nonadditivity

Ab initio Hartree-Fock (HF) and many-body perturbation theory through second (E2) and third order (E3) were used to generate pair potentials for (H₂S)₂. Several basis sets were developed by optimizing polarization functions with the zeroth-order dipole or quadrupole polarizabilities. Approximately 30 ab initio points (fully counterpoised HF + E2) in each of two larger basis sets were fit with site-site potentials. Intermediate potentials for fewer points indicate that only about 30 points are a sufficient data base to predict energies in the vicinity of the well with an accuracy of about 10%. The best ab initio well depth produced (-0.0693 eV) lies at 94% of the only published experimental value (-0.074±0.013 eV), although arguments are presented which indicate that the true (H ₂S)₂ minimum may lie below -0.08 eV. The role of E3 and nonadditive effects has also been explored, but they were found not to account for the major part of the remaining discrepancy between our best number and experiment.