Determination of a methane intermolecular potential model for use in molecular simulations from ab initio calculations

The possibility of obtaining an accurate site-site potential model suitable for use in molecular dynamics (MD) simulations of methane from ab initio calculations has been explored. Counterpoise-corrected (CPC), supermolecule, ab initio energies at the $\mathrm{MP2/6-311+G}\text{(2df,2pd)}$ level were computed for eleven relative orientations of two methane molecules as a function of C-C separation distance. C-C, C-H, and H-H interaction parameters in a pairwise-additive, site-site potential model for rigid methane molecules were regressed from the ab initio energies, and the resultant model accurately reproduced the ab initio energies. The model suggests that C-H attractions are dominant in weakly binding the methane dimer. CPC energies for methane trimers, tetramers, and a pentamer were also calculated at the same level. The results indicate that the $n$ -mer energy per pair of interactions monotonically converges with increasing $\mathrm{n,}$ but that the assumption of pairwise additivity commonly used in MD simulations is reasonably valid. A limited number of higher-level calculations using $\mathrm{MP4/6-311+G}\text{(2df,2pd)}$ and MP4/aug-cc-pVTZ were also performed to investigate the possibility of obtaining the intermolecular potential model from higher accuracy calculations without a substantial increase in computer resources. Results suggest that a Ne-methane probe method is not useful in this regard, but that limited, high-level computations, coupled with more extensive lower-level values, may be used to improve the model at minimal cost.