Determination of an ethane intermolecular potential model for use in molecular simulations from <i>ab initio</i> calculations

Abstract

Counterpoise-corrected, supermolecule, ab initio energies obtained at the MP2/6-311+G(2df,2pd) level were computed for 22 different relative orientations of two ethane molecules as a function of the separation distance between the molecular centers. These energies were used to regress the parameters in several simple, analytical, interatomic or site–site models that can be used for implementation in molecular simulations. Sensitivity analysis indicates that the intermolecular potential surface is insensitive to C–C interactions and that the parameters in the C–C model are coupled and unobtainable from the dimer energies. Representation of the potential surface can be made in terms of C–H and H–H interatomic potentials if the C–C interactions are treated as shielded. Simple Lennard-Jones and exp-6 models do not adequately represent the potential surface using these shielded models, nor do they produce the anticipated physics for the interatomic potentials. The exp-6 model with a damping function and the modified-Morse interatomic potentials both reproduce the intermolecular potential surface well with physically realistic intersite potentials suitable for use in molecular dynamics simulations.