Modification for spin-adapted version of configuration interaction singles with perturbative doubles

We propose a couple of modifications for the spin-adapted version of configuration interaction singles with perturbative doubles, originated by Head-Gordon et al. as CIS(D). A partial renormalization technique is first introduced for the T2 doubles operator of second-order Møller–Plesset perturbation (MP2) theory. Second, an extra correction from the T1 singles operator is taken into account. The combined modifications can be abbreviated as PR-CIS(Ds) (partially renormalized variant of configuration interaction singles with perturbative doubles and extra singles). A series of calculations with a benchmark set of molecules are performed for both singlet and triplet excited states. Through these tests, it is observed that favorable improvements are obtainable by the proposed modifications for low-lying valence states.