Recovering dynamic correlation in spin flip configuration interaction through a difference dedicated approach.

This article introduces the restricted-active-space n-spin flip configuration interaction models, RAS(S)-SF and RAS(S,2h,2p)-SF, which provide highly correlated, yet low cost approaches for treating polyradical systems. These levels of theory add electronic degrees of freedom beyond those of previous spin flip approaches in order to achieve accurate ground and excited state energetics. The effects of additional dynamic correlation were investigated by comparing these two techniques to the prior RAS(h,p)-SF method on a variety of test systems, including multiple electronic states of methylene, tetramethyleneethane, three binuclear transition metal complexes, and a tetracene dimer. RAS(S,2h,2p)-SF significantly improves state descriptions in all cases and provides high accuracy even when using a minimal number of spin flips. Furthermore, this correlated level of theory is shown to be extensible to the large systems involved in singlet fission, where the multi-excitonic states in tetracene dimers are difficult to simulate with standard methods and therefore are still a matter of debate. Using a triple-zeta basis, the double triplet state, (1)(TT), is predicted to be unbound. This result contradicts lower levels of theory and provides important insight into tetracene's ability to undergo efficient singlet fission.