A simple reliable approximation for isotropic intermolecular forces. A critical test using HH( 3Σ u+) as a model

Abstract

A simple semi-empirical approximation for the exchange energy ( E x), coupled with a tractable representation of the Coulomb energy ( E C), has been found to yield very accurate results for the isotropic part of the interaction energy ( E int = E x + E C) between two closed shell systems. The expression for E int is based on the knowledge of the first order Coulomb energy and the first three terms in the asymptotic long range expansion of the second order Coulomb energy for the interaction and contains but one adjustable parameter which occurs in E x. The usefulness of this approach for evaluating E int is tested critically by using the non-bonded H(1s)H(1s) ( 3Σ u +) interaction as a model (accurate values of the total interaction energy, the exchange energy, and various orders of Coulomb energies, are available for a wide range of R for this system). The results obtained for both E int and (d E int/d R) are inremarkable agreement with the exact results of Kotos and Wolniewicz for R > 3 a o. Since the law of corresponding states for inert gas pairs holds equally well for the HH( 3Σ u +) interaction, our analysis of this simple system yields valuable information on the reliability of the approach for other van der Waals dimers.