Ab initio 3D potential energy and dipole moment surfaces for the CH4-Ar complex: Collision-induced intensity and dimer content.

Abstract

We present new three-dimensional potential energy surface (PES) and dipole moment surfaces (DMSs) for the CH4-Ar van der Waals system. Ab initio calculations of the PES and DMS were carried out using the closed-shell single- and double-excitation coupled cluster approach with non-iterative perturbative treatment of triple excitations. The augmented correlation-consistent aug-cc-pVXZ (X = D,T,Q) basis sets were employed, and the energies obtained were then extrapolated to the complete basis set limit. The dipole moment surface was obtained using aug-cc-pVTZ basis set augmented with mid-bond functions for better description of exchange interactions. The second mixed virial coefficient was calculated and compared to available experimental data. The equilibrium constant for true dimer formation was calculated using classical partition function based on the knowledge of ab initio PES. Temperature variations of the zeroth spectral moment of the rototranslational collision-induced band as well as its true dimer constituent were traced with the use of the Boltzmann-weighted squared induced dipole properly integrated over respective phase space domains. Height profiles for N2-N2, N2-H2, CH4-N2, (CH4)2, and CH4-Ar true bound dimers in Titan's atmosphere were calculated with the use of reliable ab initio PESs.