Valence ab initio calculation of the potential energy curves for Ca–rare gas van der Waals molecules

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Adiabatic potential curves for the ground state and several low-lying excited states of the calcium atom interacting with rare gas (RG) atoms have been obtained in the ΛS coupling scheme from valence ab initio complete-active-space multiconfiguration self-consistent-field (CASSCF)/complete-active-space multireference second-order perturbation theory (CASPT2) calculations. In the calculations the Ca2+ and RG8+ atomic cores are represented by scalar-relativistic energy-consistent pseudopotentials. Core polarization and core-valence correlation have been accounted for by adding a core polarization potential (CPP) to the Hamiltonian. Moreover, the closed-shell single-reference coupled-cluster approach with single and double excitations including a perturbative treatment of triple excitations (CCSD(T)) has been used to obtain more accurate ground-state potential curves for the species. The transition dipole moments from the ground state to the excited 1Σ and 1Π states correlating with the (3d)1D and (4p)1P Ca terms have been evaluated as a function of R. Fine structure of the molecular terms has also been studied using a semi-empirical two-electron spin–orbit pseudopotential for the Ca atom.