Quantum mechanical and classical calculation of the transport and relaxation properties of He⋯CO2 complex using a new PES

Abstract

The intermolecular potential energy surface (PES) of He⋯CO₂ van der Waals (vdW) complex was computed at the RCCSD(T)/aug-cc-pvQz-BF level of theory. The obtained potential was fitted to an exact mathematical model using the Legendre expansion method. The fitted PES model was then used to calculate the interaction second virial coefficients (B₁₂) considering the classical and first quantum corrections, and compared with the available experimental data in the temperature range of T = 50–463.2 K. The results of the experimental and calculated B₁₂ reasonably agree. The fitted potential was also applied to compute the transport and relaxation properties of He⋯CO₂ complex from classical Mason–Monchick approximation (MMA) and Boltzmann weighting method (BWM), and the full quantum mechanical close-coupling (CC) solution of the Waldmann–Snider kinetic equation. The average absolute deviation percent (AAD%) of the experimental and CC calculated viscosity (η₁₂) and diffusion coefficients (D₁₂) were found to be 1.4% and 1.9%, respectively, which are in the range of the experimental uncertainties. However, the AAD% of MMA for η₁₂ and D₁₂ were found to be 11.2% and 11.9%, respectively. It was also found that as the temperature increased, the accuracy of MMA decreased compared to the CC method, which may be related to the elimination of the contribution of the rotational degrees of freedom, especially the off-diagonal elements in the classical MMA method. Furthermore, equilibrium classical molecular dynamics (MD) simulations based on the Green–Kubo time correlation function were performed using the Morse, LJ(12,6), and Vashishta potential models to calculate η₁₂ and D₁₂. The AAD% for η₁₂ and D₁₂ were found to be ∼13% and ∼30%, respectively, at the temperature range of T = 200–1000 K.