Eliminating the remaining spin contamination of MRSF-TDDFT by completing its core–virtual block
Year: 2026
Type: preprint
Abstract: We introduce a core–virtual-complete variant of mixed-reference spin-flip time-dependent density functional theory (MRSF-TDDFT), CV-MRSF, which completes the block of core-to-virtual (CV) excitations. For each CV excitation, the four determinants missing from MRSF-TDDFT are recovered by imposing exact spin adaptation, which makes every singlet and triplet spin pure and gives the proper singlet–triplet splitting of CV states without spin-pairing couplings. In addition, invariance under a rotation of the two singly occupied molecular orbitals (SOMOs) O1 and O2 adds the configurations C¹O₁²V¹ and C¹O₂²V¹. The number of CV configurations per excitation thus grows from one to four in the singlet and to five in the triplet configuration state function (CSF) response, and one quintet CSF also arises within the same CV block, instead of from a separate spin-flip-up calculation. As a result, the CV excitations properly couple to the complete set of open-to-open (OO) configurations, both singlet and triplet, as well as to the core-to-open (CO) and open-to-virtual (OV) configurations, which relaxes the excited states and accounts for most of the changes in excitation energy. For hybrid functionals, a spin-scalar one-body correction keeps the response matrix covariant under separate rotations of the core, open-shell, and virtual orbitals. For the Thiel benchmark set it lowers the mean absolute error of MRSF-TDDFT from 0.53 to 0.40 eV for the singlets and from 0.40 to 0.29 eV for the triplets. The largest gains are for n→π* states by excitation character and nucleobase singlets by molecule class. The CV block also describes the spin-pure triplet-pair states ¹(TT), ³(TT), and ⁵(TT) of chromophore dimers on the same footing, the essential requirement for the description of singlet fission processes.
Source: ChemRxiv
Authors: Alireza Lashkaripour, Cheol Ho Choi
Institution: Kyungpook National University
Language: English
Subfield: Atomic and Molecular Physics, and Optics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7223168231