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.
J. Phys. Chem. A
(2007), 111, pp. 5314. Accordingly, SOS-CIS(D) can be applied to the calculations of excitation energies for relatively large molecules.