Potential energy surfaces for H + Li2 → LiH + Li ground state surface from large scale configuration interaction

Abstract

Ab initio electronic structure calculations have been performed to determine the HLi₂ potential energy surface. A contracted Gaussian basis set was employed: H(5s 1p/3s 1p), Li(8s 3p/4s 3p). In addition to selfconsistent‐field (SCF) wavefunctions, full configuration interaction (CI) was carried out for the three valence electrons. For general geometry (point group C_s), the CI included 5175 configurations. For the diatomic molecules Li₂ and LiH, these methods yield dissociation energies within 5 kcal/mole of experiment, and accurate spectroscopic constants are also predicted. The minimum on the HLi₂ CI potential surface occurs for an isosceles triangle structure with r (H–Li) = 1.72 Å and on LiHLi bond angle of 95°. This minimum lies 22.4 kcal/mole below the separated products LiH + Li. The linear HLiLi minimum is much shallower, lying only 4.2 kcal/mole below the products. The much simpler single configuration SCF calculations yield qualitatively similar results. Furthermore, these features of the surface are quite analogous to those predicted for F + Li₂ by Pearson and coworkers. The angular dependence of the surface between the C_2v and C_∞v extremes is discussed. The ’’electron jump’’ from covalent HLi₂ to ionic H⁻Li₂⁺ is seen to be much more gradual than was the case for FLi₂. The electronic structure is described using a natural orbital analysis of the most important configurations in the wavefunction.