  Polarization and charge transfer are important effects which are difficult to
describe using conventional force fields. Charge equilibration models can
include both of these effects in large-scale molecular simulations. However,
these models behave incorrectly when bonds are broken, making it difficult to
use them in the context of reactive force fields. We develop a new method for
describing charge flow in molecules -- QTPIE. The QTPIE method is based on
charge transfer variables (as opposed to atomic charges) and correctly treats
asymptotic behavior near dissociation. It is also able to provide a realistic
description of in-plane polarizabilities.
