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 M(s)=0 component of the three lowest excited states (1,3)E and (3)A(1) in HCo(CO)(4) have been performed in order to evaluate the spin-orbit interactions responsible for the (1)E-->(3)E and the (1)E-->(3)A(1) radiationless transitions. A large difference is found in favor of the (1)E-->(3)E path.