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. <i>J. Chem. Theory. Comput.</i>, <b>2024</b>, <i>20</i>, 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 ¹A_g state of all-<i>E</i>-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. <i>Phys. Chem. Chem. Phys.</i>, <b>2025</b>, <i>27</i>, 12161] as well as polarizable continuum models are presented.