7.9 Correlated Excited State Methods: The CIS(D) Family

7.9.3 SOS-CIS(D) Model

As in MP2 case, the accuracy of CIS(D) calculations can be improved by semi-empirically scaling the opposite-spin components of CIS(D) expression:


with the corresponding ground state energy


More importantly, this SOS-CIS(D) energy can be evaluated with the 4th power of the molecular size by adopting Laplace transform technique.815 Accordingly, SOS-CIS(D) can be applied to the calculations of excitation energies for relatively large molecules.