Improved Complete Active Space Configuration Interaction Energies with a Simple Correction from Density Functional Theory.

Recent algorithmic advances have extended the applicability of complete active space configuration interaction (CASCI) methods to molecular systems with hundreds of atoms. While this enables simulation of photochemical dynamics in the condensed phase, the underlying CASCI method has some well-known problems resulting from a severe neglect of dynamic electron correlation. Vertical excitation energies, vibrational frequencies, and reaction barriers are systematically overestimated; these errors limit the applicability of CASCI. We develop a correction for the CASCI energy using density functional theory (DFT). The DFT correction incorporates the effect of dynamic electron correlation among the core electrons into the CASCI Hamiltonian. We show that the resulting DFT-corrected CASCI approach is applicable in situations where the usual single-reference DFT methods fail, such as the description of systems with biradicaloid electronic structure and conical intersections between ground and excited electronic states. Finally, we apply this DFT-corrected CASCI approach to ultrafast excited-state proton transfer dynamics. Without the DFT correction, CASCI predicts spurious reaction barriers to these processes, and, as a result, a qualitatively correct description of the dynamics is not possible. With the DFT-corrected CASCI method, we demonstrate qualitative and quantitative agreement with both theory and experiment for two model systems for excited-state intramolecular proton transfer. Finally, we apply the DFT-corrected CASCI method to excited-state proton transfer dynamics in a system with more than 150 atoms.