Ab initio potentials and pressure second virial coefficients for CH ₄-H₂O and CH₄-H₂S

Ab initio Hartree-Fock (HF) and second-order many-body perturbation theory (E2) were used to generate data for pair potentials of the mixtures CH ₄-H₂S and CH₄-H₂O. The points were fit with site-site functions consisting of terms for interactions of the following types: exponential repulsion and R^-6 and electrostatic attraction. The basis sets of Gaussian functions were developed by systematically adding polarization functions to maximize the zeroth-order dipole polarizability; the counterpoise method was used to correct basis set superposition error. The best well depths calculated for CH₄-H ₂S and CH₄-H₂O were - 0.0227 and -0.0215 eV, respectively (with no experimental results available for comparison). The potentials were evaluated by examining errors in the dipole polarizabilities and pressure second virial coefficients.