Unifying Charge-Flow Polarization Models

Abstract

We show that several models where electric polarization in molecular systems is modeled by charge-flow between atoms can all be considered as different manifestations of a general underlying mathematical structure. The models can be classified according to whether they employ atomic or bond parameters and whether they employ atom/bond hardness or softness. We show that an ab initio calculated charge response kernel can be considered as the inverse screened Coulombic matrix projected onto the zero-charge subspace, and this may provide a method for deriving charge screening functions to be used in force fields. The analysis suggests that some models contain redundancies, and we argue that a parameterization of charge-flow models in terms of bond softness is preferable as it depends on local quantities and decay to zero upon bond dissociation, while bond hardness depends on global quantities and increases toward infinity upon bond dissociation.

Ambiguities in Decomposing Molecular Polarizability into Atomic Charge Flow and Induced Dipole Contributions

“…Production-type polarizable force fields use the fluctuating charge model at rank-0 − and either the induced dipole − or Drude oscillator − model at the rank-1 level, some employ a combination of rank-0 and rank-1 polarization, − and a few also include quadrupole polarization. , The fluctuating charge model has nonphysical properties, such as nonlinear scaling of the polarizability with system size and charge transfer over infinite distance, but alternative charge flow models without these artifacts have received much less attention . We have shown that all charge flow polarization models can be cast into a common mathematical framework, but they differ in terms of their parametrization and numerical implementation …”