Ab initio study of the Li(2S)–H2(X 1Σg+) van der Waals complex Original Research Article

The adiabatic potential energy surface (PES) of the Li(2S)–H2(X 1Σg+) van der Waals complex, described by Jacobi coordinates (r=0.741 Å, R, Θ), was investigated using the supermolecular unrestricted fourth-order Møller–Plesset perturbation theory. Our calculations predict two equivalent minima for the linear arrangements (Θ=0° and 180°). These minima were found at R=5.22 Å with a well depth of De=68.55 μEh. This implies that Li–H2 complex is bounded very weakly. The presented PES reveals that these two minima are separated by a barrier of 25.74 μEh at Θ=90° and R=5.42 Å. The physical origin of the studied weak interaction was analysed by the intermolecular perturbation theory based on the single determinant UHF wave function. The separation of the interaction energy shows that the locations of the predicted stable structures are primarily determined by anisotropy of the repulsive Heitler–London exchange penetration and attractive dispersion+induction energy components.