An analysis of semi-empirical valence bond models for MX 2 potential energy surfaces: a case study of LiF 2

Abstract

An analysis of three semi-empirical valence-bond (VB) models of alkali-dihalide systems MX 2 is presented for the prototype molecule LiF 2. Starting with a “minimum basis” diatomics-in-molecules (DIM) Hamiltonian matrix (24 basis functions), we examine the approximations which are necessary to arrive at the simple models of Zeiri and Shapiro and Goldfield et al. and we confirm their validity within the context of the larger DIM model. The simple models owe their success to the fact that, except in some collinear configurations F:Li:F, the directional behaviour of the atomic states is of marginal importance. Also, for configurations relevant to chemical reaction, the basis functions involving the coupling of the atoms to triplet states may be safely omitted. The sensitivity of the shapes of the potential surfaces to variation of the input data for these models is also investigated.