Interaction between LiH molecule and Li atom from state-of-the-art electronic structure calculations

Abstract

State-of-the-art ab initio techniques have been applied to compute the potential energy surface for the lithium atom interacting with the lithium hydride molecule in the Born-Oppenheimer approximation. The interaction potential was obtained using a combination of the explicitly correlated unrestricted coupled-cluster method with single, double, and noniterative triple excitations [UCCSD(T)-F12] for the core-core and core-valence correlation and full configuration interaction for the valence-valence correlation. The potential energy surface has a global minimum 8743 cm^{-1} deep if the Li-H bond length is held fixed at the monomer equilibrium distance or 8825 cm^{-1} deep if it is allowed to vary. In order to evaluate the performance of the conventional CCSD(T) approach, calculations were carried out using correlation-consistent polarized valence X-tuple-zeta bases, with X ranging from 2 to 5. The contribution beyond the CCSD(T)-F12 model, obtained from full configuration interaction (FCI) calculations for the valence-valence correlation, was shown to be very small. At linear LiH-Li geometries the ground-state potential shows an avoided crossing with an ion-pair potential.Using both adiabatic and diabatic pictures we analyse the interaction between the two potential energy surfaces and its possible impact on the collisional dynamics. When the LiH bond is allowed to vary, a seam of conical intersections appears at C_{2v} geometries. At the linear LiH-Li geometry, the conical intersection is at a Li-H distance which is only slightly larger than the monomer equilibrium distance, but for nonlinear geometries it quickly shifts to Li-H distances that are well outside the classical turning points of the ground-state potential of LiH. Finally, the reaction channels for the exchange and insertion reactions are also analyzed.