Quantum Chemical Spin Densities for Radical Cations of Photosynthetic Pigment Models

Abstract

The spin densities of radical cations of magnesium porphyrin, magnesium chlorine and a truncated chlorophyll a model are calculated with density-functional theory and multiconfigurational quantum chemical methods. The latter serve as a reference for approximate density-functional theory which yields spin densities that may suffer from the self-interaction error. We carried out complete active space self-consistent field calculations with increasing active orbital spaces to systematically converge qualitatively correct spin densities. In particular, for the magnesium chlorine and chlorophyll a model radical cations, this is not easy to achieve because of the lower symmetry compared to magnesium porphyrin. Strategies had to be employed which allowed us to consider very large active orbital spaces. We explored restricted active space self-consistent field and density-matrix renormalization group calculations. Based on these reference data, we assessed the accuracy of different density-functional approximations. We show that in particular, exchange-correlation model potentials with correct asymptotic behavior yield good spin densities, and we find, in agreement with previous studies on different classes of compounds, that hybrid functionals systematically increase spin-polarization effects with increasing amounts of exact exchange. Our results provide a starting point for investigations of spin densities of more complex systems such as the hinge model for the primary electron donor in photosystem II.

Computational Investigation of the Spin-Density Asymmetry in Photosynthetic Reaction Center Models from First Principles

“…[85]. These spin densities were largely dominated by the highest occupied molecular orbital (HOMO), transforming according to the irreducible representation of point group 2 A 2 in case of chlorin and of point group 2 A for the truncated Chl a molecule [85]. The distribution in Fig.…”

“…In To provide a quantitative measure for spin-density delocalizations, a Becke-type population analysis was employed as was done in previous works [28,85]. For this purpose, accurate atom-centered Becke grids [86,87] were generated using the Serenity program [88].…”

Theoretical Assessment of Hinge-Type Models for Electron Donors in Reaction Centers of Photosystems I and II as well as of Purple Bacteria

“…Although reliable spin distributions can often be obtained for single molecules (for example, see Ref. [27]), molecular systems containing non-covalently bonded fragments, similar to those in RCs, could be especially problematic and lead to inconclusive results [26,28]. A methodology allowing to effectively avoid this limitation was reported in our recent work and called FDE-diab [28].…”

Structure of Electronically Reduced N-Donor Bidentate Ligands and Their Heteroleptic Four-Coordinate Zinc Complexes: A Survey of DFT Results

“…DFT functionals which include a small contribution from EXX are known to yield spin densities that suffer significantly from the self-interaction error. 59,104 This incorrect behavior also applies to the electronic structure of anionic radical ligands 1 •--3 •-. Concomitantly the geometry change upon reduction is related to the localization of the added electron.…”