Todd J. Martínez — Page 2

arXiv2008

QTPIE: Charge Transfer with Polarization Current Equalization. A fluctuating charge model with correct asymptotics

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.

physics.chem-ph↗

arXiv2008

A unified theoretical framework for fluctuating-charge models in atom-space and in bond-space

Our previously introduced QTPIE (charge transfer with polarization current equilibration) model (J. Chen and T. J. Martinez, Chem. Phys. Lett. 438, 315 (2007)) is a fluctuating-charge model with correct asymptotic behavior. Unlike most other fluctuating-charge models, QTPIE is formulated in terms of charge-transfer variables and pairwise electronegativities, not atomic charge variables and electronegativities. The pairwise character of the electronegativities in QTPIE avoids spurious charge transfer when bonds are broken. However, the increased number of variables leads to considerable computational expense and a rank-deficient set of working equations, which is numerically inconvenient. Here, we show that QTPIE can be exactly reformulated in terms of atomic charge variables, leading to a considerable reduction in computational complexity. The transformation between atomic and bond variables is generally applicable to arbitrary fluctuating charge models, and uncovers an underlying topological framework that can be used to understand the relation between fluctuating-charge models and the classical theory of electrical circuits.

physics.chem-ph↗