Measuring orbital interaction using quantum information theory

Quantum information theory gives rise to a straightforward definition of the interaction of electrons I_p,q in two orbitals p,q for a given many-body wave function. A convenient way to calculate the von Neumann entropies needed is presented in this work, and the orbital interaction I_p,q is successfully tested for different types of chemical bonds. As an example of an application of I_p,q beyond the interpretation of wave functions, I_p,q is then used to investigate the ordering problem in the density-matrix renormalization group.