What is the ground electronic state of KO?

High-level, restricted coupled cluster with singles, doubles, and perturbative triples calculations are performed to determine the ground electronic state of KO. In the absence of spin–orbit coupling, we find that the ground state is a S state, with a P state lying just over 200 cm higher in energy. We ascertain that basis set extension, higher-order correlation energy, mass-velocity, and Darwin relativistic terms do not change this ordering. We then calculate the low-lying V states when spin–orbit coupling is turned on. The S1/2 1 state undergoes an avoided crossing with the P1/2 state, and we therefore designate the ground state as X 1 2. This state is essentially S1/2 1 at short R, but essentially P1/2 at long R; there is a corresponding A 1 2 state with the opposite behavior. These states have significantly different shapes and so spectroscopy from the adiabatic states. Finally, we calculate the dissociation energy D0 , of KO as 6661 kcal mol 21 and derive DH f(KO, 0 K) as 13.661 kcal mol. © 2002 American Institute of Physics. @DOI: 10.1063/1.1511179#