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

7.7.3 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:

ESOS-CIS(D)=cUΨCIS|V|U2OSΨHF+cTΨCIS|V|T2OSU1ΨHF (7.39)

with the corresponding ground state energy

ESOS-MP2=cTΨHF|V|T2OSΨHF (7.40)

More importantly, this SOS-CIS(D) energy can be evaluated with the 4th power of the molecular size by adopting Laplace transform technique.778 Accordingly, SOS-CIS(D) can be applied to the calculations of excitation energies for relatively large molecules.