Ab initio intermolecular potential energy surface and second pressure virial coefficients of methane

Abstract

A six-dimensional potential energy hypersurface (PES) for two interacting rigid methane molecules was determined from high-level quantum-mechanical ab initio computations. A total of 272 points for 17 different angular orientations on the PES were calculated utilizing the counterpoise-corrected supermolecular approach at the CCSD(T) level of theory with basis sets of aug-cc-pVTZ and aug-cc-pVQZ qualities. The calculated interaction energies were extrapolated to the complete basis set limit. An analytical site-site potential function with nine sites per methane molecule was fitted to the interaction energies. In addition, a semiempirical correction to the analytical potential function was introduced to take into account the effects of zero-point vibrations. This correction includes adjustments of the dispersion coefficients and of a single-parameter within the fit to the measured values of the second virial coefficient B(T) at room temperature. Quantitative agreement was then obtained with the measured B values over the whole temperature range of the measurements. The calculated B values should definitely be more reliable at very low temperatures (T<150 K) than values extrapolated using the currently recommended equation of state.

Cross second virial coefficients and dilute gas transport properties of the (CH 4 + CO 2 ), (CH 4 + H 2 S), and (H 2 S + CO 2 ) systems from accurate intermolecular potential energy surfaces

“…In accordance with our intermolecular PESs for the pure components [3][4][5] These values correspond to the zero-point vibrationally averaged geometries. Within the rigid-rotor approximation, each configuration of two molecules can be defined by the distance between the centers of mass of the molecules, R, as well as four angles for the CH 4 -CO 2 and H 2 S-CO 2 molecule pairs and five angles for the CH 4 -H 2 S molecule pair.…”

“…In accordance with our previous work on the CH 4 -CH 4 , CO 2 -CO 2 , and H 2 S-H 2 S PESs [3][4][5], we fitted site-site potential functions with nine sites for CH 4 , seven sites for CO 2 , and 11 sites for H 2 S to the calculated interaction energies at the CCSD(T)/CBS level. Each individual site-site interaction is represented by a simple function that depends only on the distance R i j between site i in molecule 1 and site j in molecule 2,…”

“…The positions and partial charges q of the sites were taken from our previous work [3][4][5]. In our studies of the CH 4 -CH 4 [3] and CH 4 -N 2 [14] potentials, we also used zero-point vibrationally averaged monomer geometries and applied the highly accurate CCSD(T) method in combination with extrapolation to the CBS limit for the calculation of the interaction energies.…”

“…The macroscopic properties of a dilute gas are governed by binary molecular interactions, and the recent computational advances in the field have allowed us not only to accurately determine the intermolecular potential energy surfaces (PESs) using quantum-chemical ab initio methods [3][4][5][6], but also to calculate the transport properties by making use of a rigorous molecular description [7,8]. Such calculations have been performed for a number of simple gases, such as methane [9,10], carbon dioxide [5], hydrogen sulfide [4,11], water vapor [12,13], or ethylene oxide [6], yielding transport property values with an uncertainty commensurate with the best experimental measurements over wide temperature ranges.…”

“…We have previously developed such potentials [3][4][5] to investigate the thermophysical properties of these three pure gases [3][4][5][9][10][11].…”