Adaptive Configuration Interaction for Computing Challenging Electronic Excited States with Tunable Accuracy

We introduce and analyze various approaches for computing excited electronic states using our recently developed adaptive configuration interaction (ACI) method [Schriber, J. B.; Evangelista, F. A. J. Chem. Phys. 2016, 144, 161106]. These ACI methods aim to describe multiple electronic states with equal accuracy, including challenging cases like multielectron, charge-transfer, and near-degenerate states. We develop both state-averaged and state-specific approaches to compute excited states whose absolute energy error can be tuned by a user-specified energy error threshold, a. State-averaged schemes are found to be more efficient in that they obtain all of the states simultaneously in one computation, but they lose some degree of statewise tunability. State-specific algorithms allow for direct control of the error of each state, though the states must be computed sequentially. We compare each method using methylene, LiF, and all-trans polyene benchmark data.