Valence ab initio calculation of the potential-energy curves for the Ca2 dimer

The electronic structure of the Ca-2 molecule has been investigated by use of a two-valence-electron semiempirical pseudopotential and applying the internally contracted multireference configuration interaction method with complete-active-space self-consistent-field reference wave functions. Core-valence correlation effects have been accounted for by adding a core-polarization potential to the Hamiltonian. The ground-state properties of the Ca-2 and Ca-2(+) dimers have also been studied at the single-reference coupled-cluster level with single and double excitations including a perturbative treatment of triple excitations. Good agreement with experiment has been obtained for the ground-state potential curve and the only experimentally known A(1)Sigma(u)(+) excited state of Ca-2. The spectroscopic parameters D-e and R-e deduced from the calculated potential curves for other states are also reported. In addition, spin-orbit coupling between the singlet and triplet molecular states correlating, respectively, with the (4p)P-1 and (4p)P-3 Ca terms has been investigated using a semi-empirical two-electron spin-orbit pseudopotential.