Quadratic Configuration Interaction

A more acceptable way to make truncated CI size consistent was introduced by Pople et al. in 1987 [32]. Termed Quadratic Configuration Interaction (QCISD), it is formed by the addition of higher excitation terms, quadratic in the expansion coefficients, which force size-consistency. The addition of extra terms has not destroyed the correctness for two electrons (property 3 above). At the same time a perturbative treatment of the triple excitations was proposed, giving rise to QCISD(T) theory. This addition has proven to be worthwhile [37] and QCISD(TQ) has even been proposed to include quadruple excitations [38]. QCISD scales as O(N⁶) while QCISD(T) requires one iteration of O(N⁷).