A charge equilibration formalism for treating charge transfer effects in MD simulations: Application to water clusters

Abstract

Conventional classical force fields by construction do not explicitly partition intermolecular interactions to include polarization and charge transfer effects, whereas fully quantum mechanical treatments allow a means to effect this dissection (although not uniquely due to the lack of a charge transfer operator). Considering the importance of polarization in a variety of systems, a particular class of classical models, charge equilibration models, have been extensively developed to study those systems; since these types of interaction models are inherently based on movement of charge throughout a system, they are natural platform for including polarization and charge transfer effects within the context of molecular simulations. Here, we present two bond-space charge equilibration models we term as QE2 and mixed QE2 treat charge transfer in classical molecular mechanical calculations those provide practical solutions to two major drawbacks of charge equilibration models: (a) a nonvanishing amount of charge transfer between two heteroatoms at large separations, and (b) superlinear polarizability scaling during bond dissociation due to charge transfer over unphysical, large distances. To control charge transfer during dissociation of a bond in a molecular system, we introduce a distance-dependent scaling function (QE2 model) which, controls and recovers physical behavior of the homonuclear and heteronuclear charge transfer between two atoms at small and large values of internuclear separation; and the mixed QE2 model in which we combine the QE2 model under allow and disallow charge transfer situations that describe both charge transfer and polarizability in a distance-dependent manner. We demonstrate the utility of both models in the case of a water dimer, and compare the results with other existing models, and further, we perform short molecular dynamics simulations for few water clusters with the QE2 model to show the charge transfer and internuclear separation are correlated in dynamics. © 2017 Wiley Periodicals, Inc.

Self-Consistent Scheme Combining MD and Order-N DFT Methods: An Improved Set of Nonpolarizable Force Fields for Ionic Liquids

“…Including many-body interaction as charge transfer and intramolecular polarization enables these difficulties to be overcome. , Schmidt et al recently developed a polarizable force field, by using symmetry adapted perturbation theory of first-principles calculation based on density functional theory (DFT). − It provides systematic agreements for transport properties over a wide range of thermodynamic states. On the other hand, the polarizable force field technically needs to employ induced multipole moments, , dynamic charge transfer, , charge fluctuation on atoms-in-molecule, or spring-bonded charges on polarizable sites. , The number of parameters involved in potential functions then increases (sometimes drastically), and much effort may be necessary to tune those parameters which are not present in nonpolarizable force fields. Thereby, the usage of a nonpolarizable force field remains extensive as the intermediate-level molecular modeling between polarizable and coarse-grained force fields.…”

CHARMM Drude Polarizable Force Field for Glycosidic Linkages Involving Pyranoses and Furanoses

“…While the utility of empirical force fields for structural studies of carbohydrates is increasing, limitations in force fields, the most notable being the additive or fixed charge approximation, potentially limit their overall accuracy. − An important step toward overcoming this is an effort to move beyond the fixed charge force approximation by extending force fields to include explicit treatment of electronic polarization. While progress on polarizable force fields for biological molecules has been made on several fronts, − with respect to carbohydrates progress, it has been largely limited to the CHEQ fluctuating charge model and the Drude polarizable force field, with models for polyalcohols, hexopyranose and furanose monosaccharides, and acyclic monosaccharides being presented for the latter. − …”

Unifying Charge-Flow Polarization Models

“…The SQE model can be obtained by a transformation of the FQ model into a split-charge parameterization, as illustrated above, and long-distance CT between fragments can be prevented by setting all connecting t ab or p ab parameters to zero, which is equivalent to adding charge constraints for each fragment in the form of Lagrange multipliers in the FQ model. The split-charge electronegativities in L become a difference of two atomic electronegativities, and possibly including a difference in Coulombic potential arising from net charges ( Q 0 ), while the split-charge hardnesses in M ̃ become the sum of two atomic hardnesses and a κ bond hardness penalty, and these can be made distance dependent. , The p t κp split-charge penalty term together with p t Mp in eq may formally be transformed back into charge space by eq , ( J ̃ = T +t M ̃ T + ) and can be considered as κ changing the Coulombic screening function in the FQ model. We note in passing that since T + T ≠ I , a back-transformation of L and M does not produce the original χ and J ̃.…”

“…The idea of a screened Coulombic potential, and the approximate form in eq , suggests that the off-diagonal elements can be modeled by interpolating between the average diagonal elements and the R –1 Coulombic expression, as shown in eq . ,,, The value of n modulates the rate by which the Coulombic limit is approached, and the REAX force field uses n = 3 . Alternative interpolation formulas are shown in eqs and …”