Fock space coupled cluster study of the 11Πg state of the Li2 molecule

Recently reported (M. Musia, J. Chem. Phys., 136, 134111 (2012)) multireference coupled cluster method formulated in the (2,0) sector of the Fock space proved to be an efficient scheme in the treatment of potential energy curves (PECs) for alkali metal diatomics Me-2. The (2,0) sector provides description of states obtained by an attachment of two electrons to the reference which for the Me-2 is a doubly ionised Me-2(+ 2) system. The latter has a very concrete advantage in the calculations of the PECs since it dissociates into closed shell fragments (Me-2(+ 2) Me++Me+); hence, the restricted Hartree-Fock reference can be used in the whole range of interatomic distances. In this work, an accurate PEC and vibrational energy levels are obtained for the state of the lithium dimer. The average deviation of the vibrational term values from experiment is 0.79 wavenumber and is significantly lower than that provided by other theoretical works.