Spin–orbit coupled excited states in transition metal complexes: A configuration interaction treatment of HCo(CO)4

A direct spin–orbit coupled configuration interaction method is presented. The basis functions are Slater determinants expressed as combinations of alpha and beta strings. The one and two electron operators are expressed in second quantization form. Spin–orbit coupling is described as a one electron effective operator. The structure of the configuration expansion is single and double excitations from a limited full CI space. Calculations of the Ms=0 component of the three lowest excited states 1,3E and 3A1 in HCo(CO)4 have been performed in order to evaluate the spin–orbit interactions responsible for the 1E→3E and the 1E→3A1 radiationless transitions. A large difference is found in favor of the 1E→3E path.