Method of Diatomics in Molecules. V. Theoretical Prediction of Stable Li2H+ and Almost Stable LiH2+

Diatomics-in-molecules theory is applied to the molecule—ions LiH2+ and Li2H+ using as a basis three canonical valence-bond wavefunctions in each case. The input for this theory consists of the valence-bond wavefunctions and potential-energy curves for the ground electronic states of H2, Li2, and LiH and for the ground and first excited states of H2+, Li2+, and LiH+. Experimental diatomic potential-energy curves are used if available; otherwise, best possible theoretical or semiempirical curves are used. Diatomics-in-molecules theory in this approximation leads to predictions that ground-state LiH2+ may be stable by about 2.5 kcal with respect to Li++H2 and that ground-state Li2H+ should be stable by about 10 kcal with respect to Li++LiH. The equilibrium shape for LiH2+ turns out to be a symmetrical isosceles triangle with RLiH=5.0 bohrs and RHH=1.4 bohrs; Li2H+ is symmetrical and linear with RLiH=3.4 bohrs. A stable linear excited state (1Σu+) is predicted at about 35 kcal above the ground state (1Σg+) of Li2H+.