We study the electronic structure and correlations of vitamin B12
(cyanocobalamine) by using the framework of the multi-orbital single-impurity
Haldane-Anderson model of a transition-metal impurity in a semiconductor host.
The parameters of the effective Haldane-Anderson model are obtained within the
Hartree-Fock (HF) approximation. The quantum Monte Carlo (QMC) technique is
then used to calculate the one-electron and magnetic correlation functions of
this effective model. We observe that new states form inside the semiconductor
gap found by HF due to the intra-orbital Coulomb interaction at the impurity 3d
orbitals. In particular, the lowest unoccupied states correspond to an impurity
bound state, which consists of states from mainly the CN axial ligand and the
corring ring as well as the Co e_g-like orbitals. We also observe that the
Co(3d) orbitals can develop antiferromagnetic correlations with the surrounding
atoms depending on the filling of the impurity bound states. In addition, we
make comparisons of the HF+QMC data with the density functional theory
calculations. We also discuss the photoabsorption spectrum of cyanocobalamine.Comment: 20 pages, 14 figure
