Nonorthogonal Configuration Interaction of Constraint-Based Orbital-Optimized Excited States: A Versatile Method for Theoretical Photochemistry

We introduce a nonorthogonal configuration interaction (NOCI) scheme for COOX, our recently developed constraint-based orbital-optimized excited state method [Kussmann et al. J. Chem. Theory. Comput., 2024, 20, 8461], which enables a targeted variational optimization of electronically excited states through constrained density functional theory. COOX is shown to be a more reliable source of NOCI reference configurations compared to orbital-optimized methods based on the ΔSCF scheme. The versatility and stability of NOCI-COOX are illustrated for the 2 1Ag state of all-E-polyenes, conical intersections, core excitations, and other cases that are challenging to traditional linear-response time-dependent DFT approaches, and exemplary calculations for the NOCI-COOX treatment of photoactive species in complex molecular or bulk environments by virtue of our recent e-COOX embedding scheme [Lemke et al. Phys. Chem. Chem. Phys., 2025, 27, 12161] as well as polarizable continuum models are presented.