For the sake of our explanation, we turn our attention back to the total energy of the excited state at the CIS(D) level. In the same way that the total ground-state energy can be broken down into the Hartree-Fock total energy and the MP2 correlation energy, the total excited state energy for CIS(D) can be broken down into the total excited state energy at the CIS level and the electron correlation at the CIS(D) level E CIS(D) C . Formally, the latter is defined as: