  One of the most critical tasks at the very beginning of a quantum chemical
investigation is the choice of either a multi- or single-configurational
method. Naturally, many proposals exist to define a suitable diagnostic of the
multi-configurational character for various types of wave functions in order to
assist this crucial decision. Here, we present a new orbital-entanglement based
multi-configurational diagnostic termed $Z_{s(1)}$. The correspondence of
orbital entanglement and static (or nondynamic) electron correlation permits
the definition of such a diagnostic. We chose our diagnostic to meet important
requirements such as well-defined limits for pure single-configurational and
multi-configurational wave functions. The $Z_{s(1)}$ diagnostic can be
evaluated from a partially converged, but qualitatively correct, and therefore
inexpensive density matrix renormalization group wave function as in our
recently presented automated active orbital selection protocol. Its robustness
and the fact that it can be evaluated at low cost make this diagnostic a
practical tool for routine applications.
