  Fluctuating-charge models have been used to model polarization effects in
molecular mechanics methods. However, they overestimate polarizabilities in
large systems. Previous attempts to remedy this have been at the expense of
forbidding intermolecular charge-transfer. Here, we investigate this lack of
size-extensivity and show that the neglect of terms arising from charge
conservation is partly responsible; these terms are also vital for maintaining
the correct translational symmetries of the dipole moment and polarizability
that classical electrostatic theory requires. Also, QTPIE demonstrates
linear-scaling polarizabilities when coupling the external electric field in a
manner that treats its potential as a perturbation of the atomic
electronegativities. Thus for the first time, we have a fluctuating-charge
model that predicts size-extensive dipole polarizabilities, yet allows
intermolecular charge-transfer.
