Quantum states entanglement in hemoglobin molecule active center
  An ab initio study of the electronic and spin configuration for the iron ion in the active center of the human hemoglobin molecule is presented. It is well known that the iron ion, being surrounded by the porphyrin ring and the ligands, plays the key role in the realization of the basic oxygen-transport functions of the molecule. This work is focused on the investigation the features of the 3$d$-shell electronic states of the iron ion located inside the active center of the hemoglobin molecule. Also in this paper we study in detail the changes in these states occurring during the oxidation process. We use a combination of the Density Functional Theory (DFT) method and the Dynamical Mean Field Theory (DMFT) approach. This method allows to consider dynamic correlation effects that are important in the description of systems containing transition metal ions. It was found that the state of the valence electrons of the iron ion of the active center of hemoglobin molecule is the entangled quantum state. This state is a mixture of several electronic states with comparable statistical probability. Furthermore, it was found that the process of the bond formation between the iron-porphyrin complex and the oxygen molecule is more complex than a simple high-spin to low-spin state of the Fe ion transition. The transition metal ion oxidation is accompanied by substantial redistribution of the states probabilities and the increasing of the entanglement degree. This process also leads to the reduction of the total spin moment from $s\approx$2.1 for the FeP(Im) to $s\approx$1.7 for the FeP(Im)(O$_2$). 
D. Novoselov, Dm. M. Korotin, V. I. Anisimov
cond-mat.str-el