Exact entanglement correlation complements the chemical bond description

We analyze the properties of the exact solution obtained by us recently for the extended Heitler-London model of the chemical bonding in an analytic form. The emphasis, put on defining the two-particle entanglement correlation, and expressing it by the von Neumann entropy, which provides the complementary characterization of the chemical bond and is directly related to the partial atomicity and the true covalency. The newly introduced concepts refine the standard analysis of covalency based solely on the wave-function and related energy description of the bond as a pure state. We show that a gradual evolution of the system with the increasing interatomic distance of the indistinguishable two electrons composing the bond into their distinguishable correspondents in the atomic (dissociation) limit can be traced out systematically. In particular, only by taking into account the atomicity the true covalency vanishes in the latter limit.