While it might appear that double excitations are crucial for accurately modeling liquid-phase XAS, the relative contribution of doubly-excited determinants may be significantly lower for other choices of reference orbitals. In the ground state, the electron correlation energy, E c , is defined as the difference between the exact energy and a mean-field reference E 0 : E c = E -E 0 . If there are multiple choices for E 0 , then E c is not uniquely defined and one must be specific (e.g. using spin-polarized versus spin-restricted orbitals). Such considerations are even more vital for core excitations, which create a core hole at a specific atomic site, and reattach the electron into an empty valence orbital. One could create an open-shell configuration using ground-state orbitals Φ * 0 with energy E * 0 (0), yielding E * c (0) = E * -E * 0 (0). Or, one could make a much better determinant by minimizing the energy E * 0 of the open-shell configuration, yielding state-specific optimized orbitals Φ * (opt) and a much smaller correlation energy