Coupled-Cluster Study of Spectroscopic Constants of the Alkali Metal Diatomics: Ground and the Singlet Excited States of Na2, NaLi, NaK, and NaRb

A newly implemented two-determinant coupled-cluster method including single and double excitation operators (TD-CCSD) is applied to calculations of spectroscopic constants of alkali metal diatomics. The equilibrium bond length, harmonic vibrational frequency, anharmonicity, the dissociation energy and excitation energies are derived from the potential curves calculated for the ground state X 1 Σ + u and the two singlet mono-excited states, A 1 Σ + u and B 1 Π u of Na 2 as well as for the ground X 1 Σ + and the singlet excited A 1 Σ + and B 1 Π states of the heteronuclear NaLi, NaK, and NaRb molecules. Spectroscopic constants and excitation energies agree reasonably well with experiment. Our results demonstrate that the relatively simple CCSD method for the excited states represented by two-reference determinants is a viable technique. The computer time needed for an excited singlet state is practically identical to the time which a standard single-determinant-based CCSD calculation takes.