Ground State and Heat of Formation of MgB2

A survey of the low-lying states of the MgB2 and MgBB molecules is made, and it is found that different levels of theory lead to different calculated ground states. By employing high-level, up to RCCSD(T)/aug-cc-pCVQZ, ab initio calculations, we conclude that the lowest energy structure is a T-shaped X̃A1 state, but that there is a ã3B1 state lying within 1 kcal mol-1 higher in energy, with 3Σ- and 5Σ- MgBB states lying ∼12 kcal mol-1 above that. The ground state is confirmed at the CASSCF+MRCI+Q level. We also calculate ΔH f[MgB2(X̃1A1)], obtaining a value of 194 ± 5 kcal mol-1.