Department of Chemical Physics and Optics, Faculty of Mathematics and Physics, Charles University, Ke Karlovu 3, 12000 Prague, Czech Republic.
Mutlireference perturbation theory (CAS-PT2), random phase approximation (RPA), configuration interaction with singles (CIS) (all at various basis sets) and semiempirical ZINDO methods were used for the determination of the excited states energies of free-base porphin and Mg-porphin. On the basis of these results an error estimation of the methods used for larger systems (molecules in PSII reaction center) calculation can be established.
The PSII model in ref. (1) was used for the electron-exication spectra determination. First, individual ZINDO spectra of monomers chlorophyll-a and pheophytine-a were estimated for semiempirically (PM3) fully optimized structures.
As a next step, partial optimizations (the hydrogen atoms only) on all the monomers, selected dimers, trimers, tetramers, and hexamer were done at the same PM3 semiempirical level, keeping the positions of all the “x-ray determined atoms“ fixed. Selection of studied dimeric and oligomeric structures is based on the considerations of the multiparticle (multimolecular) excitations. These multiparticle specta exhibits complex non-additive character where all the basic spectra lines (Qx, Qy, Soret lines) are shifted unevenly from their positions in isolated molecules. This uneven character speaks out about the different importance of individual molecules in the various excitations.
Anothe comparison of electron-excitation spectra using ZINDO and RPA was performed for the chlorophyll-a and pheophytine-a molecules, as well. For deeper elucidation, the structures used in comparison were both fully-optimized and “hydrogens-optimized“ (taken from RC model).
(1) S. Ruffle, D. Donnelly, T. Blundell, J. Nugent, A 3-Dimensional model of the photosystem-II reaction center of Pisum-Sativum, Photosynthesis research, 34 (1992), 287-300.