  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.
