A study of Be2 with many-body perturbation theory and a coupled-cluster method including triple excitations

The unusual potential energy curve for the 1∑+g ground state of Be2 is investigated using many-body perturbation theory (MBPT) and coupled-cluster (CC) methods. The curve, which has a ∼2 kcal/mol inner minimum at ∼2.6Å and a van der Waals minimum at ∼5.0 Å, is very difficult to describe accurately with even high-level ab initio correlated methods. To resolve uncertainties in previous MBPT/CC studies, we have generalized CC theory to include effects of triple excitations. The present calculations are compared with recent full CI results to assess the relative importance of different contributions of electron correlation. MBPT (4) is found to be qualitatively correct, but to slightly exceed the correct full CI binding energy, while CC theory even with triple excitations, has no inner minimum. The latter follows from CC theory being exact for separated Be atoms (with frozen core) but to have a 1% correlation energy error in the binding region. Yet this 1% accounts for the ∼2 kcal/mol inner well. The possibility of the remaining 1% arising from connected quadruple excitations T4 is discussed.