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,
σ. 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.