A Constraint-Based\nOrbital-Optimized Excited State\nMethod (COOX)

In this work, we present a novel method to directly calculate\ntargeted\nelectronic excited states within a self-consistent field calculation\nbased on constrained density functional theory (cDFT). The constraint\nis constructed from the static occupied-occupied and virtual-virtual\nparts of the excited state difference density from (simplified) linear-response\ntime-dependent density functional theory calculations (LR-TDDFT).\nOur new method shows a stable convergence behavior, provides an accurate\nexcited state density adhering to the Aufbau principle, and can be\nsolved within a restricted SCF for singlet excitations to avoid spin\ncontamination. This also allows the straightforward application of\npost-SCF electron-correlation methods like MP2 or direct RPA methods.\nWe present the details of our constraint-based orbital-optimized excited\nstate method (COOX) and compare it to similar schemes. The accuracy\nof excitation energies will be analyzed for a benchmark of systems,\nwhile the quality of the resulting excited state densities is investigated\nby evaluating excited state nuclear forces and excited state structure\noptimizations. We also investigate the performance of the proposed\nCOOX method for long-range charge transfer excitations and conical\nintersections with the ground-state.