CIS(D) is a simple, size-consistent doubles correction to CIS which has a computational cost scaling as the fifth power of the basis set for each excited state.440, 436 In this sense, CIS(D) can be considered as an excited state analog of the ground state MP2 method. CIS(D) yields useful improvements in the accuracy of excitation energies relative to CIS, and yet can be applied to relatively large molecules using Q-Chem’s efficient integrals transformation package. In addition, as in the case of MP2 method, the efficiency can be significantly improved through the use of the auxiliary basis expansions (Section 6.6).961