Nature of interaction energy anisotropy in the Li(2S)–HF (˜X1Σ+) van der Waals complex. A theoretical study

Abstract

The potential-energy surface for the Li(2S)–HF (˜ X1Σ+ interaction, where HF is kept rigid, is calculated using the supermolecular unrestricted fourth-order Moller–Plesset perturbation theory. The basis set superposition error corrected potential indicates two minima. The global minimum occurs for the bent Li...FH structure at R=1.95 A and θ=70° with a relatively deep well of De=1,706 cm−1 and the secondary minimum is found for the linear Li...HF configuration at R=4.11 A with a well depth ofDe=288 cm−1. A barrier of 177 cm−1 (with respect to the secondary linear minimum) separates these two minima. In this study 27 bound states of the bent Li...FH minimum and eight bound states of the linear Li...HF minimum up to the Li+HF dissociation threshold are calculated. The energy partitioning using the intermolecular perturbation theory scheme shows that the origins of the stability of the structures studied are entirely different. The global minimum is stabilised using the attractive Coulombic interaction and unrestricted Hartree–Fock deformation energy. The latter term originates from the mutual electric polarisation effects. The secondary linear minimum is mostly determined by the anisotropy of the repulsive Heitler–London exchange-penetration and attractive dispersion energies.