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J Chem Phys. 2013 Jun 14;138(22):221105. doi: 10.1063/1.4809571.

Communication: an inexpensive, variational, almost black-box, almost size-consistent correction to configuration interaction singles for valence excited states.

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Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA.


Configuration interaction singles (CIS) describe excited electronic states only qualitatively and improvements are imperative as a means of recovering chemical accuracy. In particular, variational improvements would be ideal to account for state crossings and electronic relaxation. To accomplish such an objective, in this communication we present a new suite of algorithms, abbreviated VOO-CIS for variationally orbital optimized CIS. We show below that VOO-CIS yields a uniform improvement to CIS, rebalancing the energies of CT states versus non-CT states within the same framework. Furthermore, VOO-CIS finds energetic corrections for CT states that are even larger than those predicted by CIS(D). The computational cost of VOO-CIS depends strongly on the number of excited states requested (n), but otherwise should be proportional to the cost of CIS itself.

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