Thus far, coupled cluster singles and doubles (CCSD), multi-reference configuration interaction (MRCI), and multi-reference second-order perturbation theory (MRPT2) have been applied to this approach. However, these approximations may not be accurate when the SO interaction is substantial or when the number of SF states considered in this approach may be insufficient. A more direct method to incorporate the SO interaction is to determine one-electron molecular orbitals using a spin-dependent Hamiltonian during the self-consistent field (SCF) calculations, such as with the four-component Dirac or the two-component quasi-relativistic Hamiltonians.