Ab Initio Prediction of Spectroscopic Constants of CaN in the Lowest 2, 4-, 2-, and 4 Electronic States: A Potential Candidate for the First Calcium-bearing Interstellar Molecule

Spectroscopic constants for the lowest 2, 4-, 2-, and 4 states of the CaN radical have been calculated by the multireference singles and doubles configuration interaction method, including Davidsonʹs correction for quadruple excitations [MR-SDCI(+Q)], to aid in detection of the first Cabearing molecule in interstellar space. It is shown that the 2 state is the ground state of CaN and that the 4- state is the first excited state, although their energy difference is very small (493.4 cm-1). This result is contrary to that for the isovalent species MgN. The calculated rotational constants for the 2 state are Be = 10.767 GHz and B0 = 10.731 GHz, while those for the 4- state are Be = 9.3807 GHz and B0 = 9.3373 GHz.