In state of the art EE approaches, the QM electron density is optimized accounting for the electric field generated by the MM system. In this context, following the same notation than before, the QM energy in equation ( ), E QM , should be noted as E sol QM . While the polarization energy from equation ( ) represents a correction of the QM/MM interaction energy at the ME level, we are still omitting the energy cost of polarizing the QM region.