Effects of exchange energy and orbital orthogonality on barriers to internal rotation

Abstract

In order to determine the effect of the Pauli principle on internal rotation barriers, we have calculated exchange and orbital orthogonality contributions to barriers for a small group of molecules: ethane, methanol, acetaldehyde, and hydrogen peroxide. To evaluate exchange contributions it was necessary to formulate variation equations and to energy-optimize simple product wavefunctions of orthogonal orbitals (Hartree-product wavefunctions). The resulting orbitals resemble very closely the Edmiston–Ruedenberg localized Hartree–Fock orbitals. Our results indicate that exchange contributions to the barriers in ethane and acetaldehyde are quite small. For methanol and hydrogen peroxide, our calculated contributions are considerably larger and suggest that, at least in some molecules, exchange contributions cannot be ignored. For all of the molecules, however, the exchange contribution is greatly overshadowed by that of end-to-end orbital orthogonality.