Natural bond orbital analysis of steric interactions

We describe an ab initio procedure for extracting the Pauli exchange antisymmetry (“steric”) contributions to molecular potential energy in the framework of self-consistent-field molecular orbital (SCFMO) theory. This “natural steric analysis” method is based on natural bond orbital (NBO) representation of the SCFMO wave function, which allows the steric exchange energy to be approximated as an energy difference between “preorthogonal” and final NBOs, analogous to the procedure of Sovers et al. [J. Chem. Phys. 49, 2592 (1965)]. We show how the total NBO steric exchange energy can in turn be approximated in terms of pairwise-additive interactions between localized bonding units, comparable to the empirical steric potentials of molecular mechanics models. The accuracy of NBO steric analysis is tested in applications to various rare-gas interactions (expected to be of pure steric exchange type), and excellent agreement is found with the full ab initio potential curves over a wide range of separations. Limitations of the simple pairwise-additive model of steric exchange interactions are noted and characterized.