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.