Quest for the ground state characterization of CaC

The ground state of CaC is determined from high-order coupled cluster calculations to be a 3Σ− state with a rotational constant of 9.94 GHz. This state lies close to a 5Σ− state.Lower-order calculations give the 5Σ− state below but at the highest level of calculation, CCSD(T), the 3Σ− state lies about 800 cm−1 below the 5Σ− state. This reproduces the situation of similar diatomics such as B2, BC+, BeC and MgC where similar competition between the 3Σ− and 5Σ− states has been solved in favor of 3Σ− as the ground state. High-order single and double excitations are crucial but further adding triple excitations is decisive to give the proper ordering of these two states. The use of the frozen core approximation does not change this qualitative pattern but gives bond distances that are too long making this approximation somewhat unreliable for predicting the microwave spectrum necessary for a proper identification of this astrophysically important carbide. A broken symmetry spin-contaminated triplet solution is found that lies lowest at every level of calculation considered.