  Variable charge models (e.g., EEM, QEq, ES+) in reactive molecular dynamics
simulations often inherently impose a global charge transfer between atoms
(approximating each system as ideal metal). Consequently, most surface
processes (e.g., adsorption, desorption, deposition, sputtering) are affected,
potentially causing dubious dynamics. This issue is meant to be addressed by
the ACKS2 and QTPIE model, which are based on the Kohn-Sham density functional
theory as well as a charge transfer restricting extension to the QEq model
(approximating each system as ideal insulator), respectively. In a brief review
of the QEq and the QTPIE model, their applicability for studying surface
interactions is assessed in this work. Following this reasoning, the demand for
a revised generalization of the QEq and QTPIE model is proposed, called charge
transfer equilibration model or in short QTE model. This method is derived from
the equilibration of constrained charge transfer variables, instead of
considering atomic charge variables. The latter, however, are obtained by a
respective transformation, employing an extended Lagrangian method. We moreover
propose a mirror boundary condition and its implementation to accelerate
surface investigations. The models proposed in this work facilitate reactive
molecular dynamics simulations which describe various materials and surface
phenomena appropriately.
