Stability of Hyperlithiated Borides

Structures and energetics of BLi_n (n = 4-8) clusters are predicted using the SCF, MP2, and B3LYP methods with the 6-31G(d) basis set, including energy evaluations at G2MP2. Cohesive energies, defined as the enthalpies of the BLi_n → B + Li_n reactions, and Li and Li₂ elimination reaction enthalpies are also estimated at B3LYP. This level of theory predicts the boron cohesive energy to increase up to the BLi₆ cluster after which it levels off. All BLi_n systems are thermodynamically stable with respect to Li and Li₂ dissociations; BLi₄ has the largest reaction enthalpies. The energetics of the hyperlithiated borides obtained with B3LYP/ 6-31G(d) are in reasonable agreement with those at G2MP2 but less satisfactory than those of the smaller BLi_n (n = 1-3) systems. Computations on BLi₄ with multiconfigurational quasidegenerate perturbation theory indicate that the B3LYP/6-31G(d) energies may be more reliable for the larger BLi_n systems.