  Quantum-chemical multi-configurational methods are required for a proper
description of static electron correlation, a phenomenon inherent to the
electronic structure of molecules with multiple (near-)degenerate frontier
orbitals. Here, we review how a property of these frontier orbitals, namely the
entanglement entropy is related to static electron correlation. A subset of
orbitals, the so-called active orbital space, is an essential ingredient for
all multi-configurational methods. We proposed an automated selection of this
active orbital space, that would otherwise be a tedious and error prone manual
procedure, based on entanglement measures. Here, we extend this scheme to
demonstrate its capability for the selection of consistent active spaces for
several excited states and along reaction coordinates.
