XVIth International Workshop on
Quantum Systems in
Chemistry and Physics
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Towards reliable density functional approaches for manganese complexes
Shusuke Yamanaka,1 Keita Kanda,1 Toru Saito,1 Yasutaka Kitagawa,1
Takashi Kawakami,1 Masahiro Ehara,2 Mitsutaka Okumura,1
Haruki Nakamura,3 Kizashi Yamaguchi4

1Graduate School of Science, Osaka University, Japan
2Institute for Molecular Science, Japan
3Protein Institute, Osaka University, Japan
4TOYOTA Physical & Chemical Research Institute, Japan,

I. Introduction
Recently we have investigated a manganese cluster, a model for the oxygen evolving complex (OEC) in photosytem II [1], which catalyze the decomposition reaction of water. We rely on the newest and most reliable X-ray structure [2] and the optimized ones starting from the X-ray structure. However the stable spin states for some of them calculated by the B3LYP method are not consistent with the EPR experimental results of the native cluster [1]. We are now examining the modeling structure including various protonation types to the oxygens in-and-around the cluster. It is also possible that the B3LYP does yield incorrect magnetism of the manganese cluster. To confirm this, we implement a benchmark test of B3LYP, as well as other functionals, for calculations of magnetism of various manganese complexes [3].
II. Results
For a test set, we choose dinuclear manganese complexes, for which the X-ray diffraction structure and the magnetic interaction have been experimentally reported. It includes various manganese complexes with oxidation patterns as follows: Mn(II)2 Mn(II)Mn(III), Mn(III)2, Mn(III)Mn(IV), and Mn(IV)2. First, starting from triple-zeta plus diffuse and polarization function quality, we pruned the basis set step by step with monitoring computational results of magnetic interactions using B3LYP functional, deciding a modest but reliable basis set. With employing the basis set, we examined various exchange-correlation functionals, BHandHLYP, CAM-B3LYP, TPSSh, PBE0, PBEh35, HSEh1PBE, HSE1PBE, HSE2PBE, LC-wPBE, M06, M06-2X, M06-HF, HF, B3LYP*, for calculations of magnetism. The computational results will be discussed in relation to the experimental results. In particular we focus our attention on the effects of protonation to bridged oxygens and changes of oxidation states on the magnetic interactions, which will be a guideline principle to inspect the protonation modes of states in the Kok cycle of OEC.

References
[1] K. Kanda et al. CPL 506 (2011) 98; S. Yamanaka, et al. CPL 511(2011)138; K. Kanda et al. Polyhedron in press; S. Yamanaka et al. Adv Quantum Chem, submitted.
[2] Y. Umena, K. Kawakami, J-R. Shen, N. Kamiya, Nature 473 (2011) 55.
[3] S. Yamanaka et al., in preparations.


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