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

Yannick Lemke¹, Jörg Kussmann¹, Christian Ochsenfeld^1,2

¹Chair of Theoretical Chemistry, Department of Chemistry, Ludwig-Maximilians-Universität München, D-81377 Munich, Germany.

Journal of Chemical Theory and Computation

|October 8, 2025

Abstract

None:

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 ¹A_g 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.

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