2026/05/31 by Xiaoyu Zhang, Tai Wang
Physics and Astronomy · #physics.chem-ph
15 pages, 3 figures, 4 tables, 66 equations
arxiv created 2026/07/31 · arxiv updated 2026/08/03
Linear-response TDDFT based on open-shell references may produce spin-mixed excited states, particularly when the response space is not spin complete. Spin-adapted RPA was developed using tensor equation-of-motion and applying the Wigner-Eckart theorem with tensor decoupling. Casting the RPA Fock matrix and kernels as energy derivatives gives a TDDFT extension. To restore spin-component degeneracy and eliminate multi-counting of spin correlation in pure exchange-correlation functionals, we use hybrids combining HF exchange with spin-unpolarized pure XC parts. Given that the mapping from spin-adapted RPA to spin-adapted TDDFT is ad hoc, we introduce a first-principles framework for evaluating the expectation value ⟨ S2 ⟩ of excited states in order to rigorously prove their spin purification. In addition, we derive explicit working equations for the computation of the single-particle density matrix of excited states, thereby enabling a natural orbital analysis of their electronic structure. Benchmarks, diradicals/triradicals, Cr2 dissociation, and phenol O-H conical intersections demonstrate the method.