  We study the electronic structure and the magnetic correlations of
cyanocobalamin ($C_{63}H_{88}CoN_{14}O_{14}P$) by using the framework of the
multi-orbital single-impurity Haldane-Anderson model of a transition metal
impurity in a semiconductor host. Here, we first determine the parameters of
the Anderson Hamiltonian by performing density functional theory (DFT)
calculations. Then, we use the quantum Monte Carlo (QMC) technique to obtain
the electronic structure and the magnetic correlation functions for this
effective model. We find that new electronic states, which correspond to
impurity bound states, form above the lowest unoccupied level of the host
semiconductor. These new states derive from the atomic orbitals at the cobalt
site and the rest of the molecule. We observe that magnetic moments develop at
the Co($3d_{\nu}$) orbitals and over the surrounding sites. We also observe
that antiferromagnetic correlations exist between the Co($3d_{\nu}$) orbitals
and the surrounding atoms. These antiferromagnetic correlations depend on the
filling of the impurity bound states.
