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Electronic structure and correlations of vitamin B12 studied within the Haldane-Anderson impurity model
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We study the electronic structure and correlations of vitamin B12 (cyanocobalamine) by using theframework of the multi-orbital single-impurity Haldane-Anderson model of atransition-metal impurity in a semiconductor host. The parameters of the effectiveHaldane-Anderson model are obtained within the Hartree-Fock (HF) approximation. Thequantum Monte Carlo (QMC) technique is then used to calculate the one-electron andmagnetic correlation functions of this effective model. We observe that new states forminside the semiconductor gap found by HF due to the intra-orbital Coulomb interaction atthe impurity 3d orbitals. In particular, the lowest unoccupiedstates correspond to an impurity bound state, which consists of states from mainly the CNaxial ligand and the corrin ring as well as the Co eg-like orbitals. We alsoobserve that the Co (3d) orbitals can develop antiferromagneticcorrelations with the surrounding atoms depending on the filling of the impurity boundstates. In addition, we make comparisons of the HF+QMC data with the density functionaltheory calculations. We also discuss the photoabsorption spectrum of cyanocobalamine.