To study charge transfer between a molecule and a metal surface, the active space must be chosen carefully. In general, the 2 active orbitals t, u must be well-balanced mixtures of both the impurity atomic orbitals {d µ } and the bath atomic orbitals {b ν }. We will not be able to recover charge transfer if the active orbitals are exclusively on the impurity or in the bath. Thus, a constraint becomes necessary if we wish to guarantee that we find such a well-balanced mixture of orbitals from a CASSCF solution.