Relaxation and correlation contributions to molecular double core ionization energies

We describe self-consistent field optimization of double core hole states for single- (SCF) and multi-configuration (MCSCF) wavefunctions. With the proposed schemes it is possible to separate contributions to the double core hole ionization energies due to relaxation from those due to correlation, and to obtain, optionally, both symmetry broken and symmetry adapted solutions of such states. Sample calculations are performed to elucidate the role of single versus double core hole relaxation and static versus residual relaxation when the two core holes are in the same main shell but on different sites and when they are on the same site but in different main shells. Perturbation theory results for the double equivalent core replacement species are presented and discussed. The "singlet-triplet separability problem" previously identified for certain double core hole states is verified and remediated by second order SCF/MCSCF.