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

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. (C) 2001 American Institute of Physics.