title: Spin densities from subsystem density-functional theory: assessment and application to a photosynthetic reaction center complex model.
abstract: Subsystem density-functional theory (DFT) is a powerful and efficient alternative to Kohn-Sham DFT for large systems composed of several weakly interacting subunits. Here, we provide a systematic investigation of the spin-density distributions obtained in subsystem DFT calculations for radicals in explicit environments. This includes a small radical in a solvent shell, a π-stacked guanine-thymine radical cation, and a benchmark application to a model for the special pair radical cation, which is a dimer of bacteriochlorophyll pigments, from the photosynthetic reaction center of purple bacteria. We investigate the differences in the spin densities resulting from subsystem DFT and Kohn-Sham DFT calculations. In these comparisons, we focus on the problem of overdelocalization of spin densities due to the self-interaction error in DFT. It is demonstrated that subsystem DFT can reduce this problem, while it still allows to describe spin-polarization effects crossing the boundaries of the subsystems. In practical calculations of spin densities for radicals in a given environment, it may thus be a pragmatic alternative to Kohn-Sham DFT calculations. In our calculation on the special pair radical cation, we show that the coordinating histidine residues reduce the spin-density asymmetry between the two halves of this system, while inclusion of a larger binding pocket model increases this asymmetry. The unidirectional energy transfer in photosynthetic reaction centers is related to the asymmetry introduced by the protein environment.
authors.affiliation: Gorlaeus Laboratories, Leiden Institute of Chemistry, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands.
authors.forename: Alisa
authors.initials: A
authors.lastname: Solovyeva
authors.forename: Michele
authors.initials: M
authors.lastname: Pavanello
authors.forename: Johannes
authors.initials: J
authors.lastname: Neugebauer
mesh_terms: D001429:Bacteriochlorophylls / Q000737:chemistry*; D005609:Free Radicals / Q000737:chemistry*; D008962:Models, Theoretical; D015394:Molecular Structure; D010788:Photosynthesis; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry*; D020560:Proteobacteria / Q000737:chemistry*; D012997:Solvents / Q000737:chemistry; D019097:Spin Trapping
chemical_list: D001429:Bacteriochlorophylls; D005609:Free Radicals; D045322:Photosynthetic Reaction Center Complex Proteins; D012997:Solvents