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

The electronic structure of the Ca2 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 Ca2 and Ca2+ 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 A1Σu+ excited state of Ca2. The spectroscopic parameters De and Re 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)1P and (4p)3P Ca terms has been investigated using a semi-empirical two-electron spin–orbit pseudopotential.