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 A¹_u⁺ excited state of Ca₂. 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)¹P and (4p)³P Ca terms has been investigated using a semi-empirical two-electron spin–orbit pseudopotential.Acknowledgement.This work was supported by grant 5 P03B 082 21 from the Polish State Committee for Scientific Research (KBN).