Continuum level treatment of electronic polarization in the framework of molecular simulations of solvation effects

Abstract

The hybrid molecular–continuum model for polar solvation considered in this paper combines the dielectric continuum approximation for treating fast electronic (inertialess) polarization effects and a molecular dynamics (MD) simulation for the slow (inertial) polarization component, including orientational and translational solvent modes. The inertial polarization is generated by average charge distributions of solvent particles, composed of permanent and induced (electronic) components. MD simulations are performed in a manner consistent with the choice of solvent and solute charges such that all electrostatic interactions are scaled by the factor 1/ε∞, where ε∞ is the optical dielectric permittivity. This approach yields an ensemble of equilibrium solvent configurations adjusted to the electric field created by a charged or strongly polar solute. The electrostatic solvent response field is found as the solution of the Poisson equation including both solute and explicit solvent charges, with accurate account of electrostatic boundary conditions at the surfaces separating spatial regions with different dielectric permittivities. Both equilibrium and nonequilibrium solvation effects can be studied by means of this model, and their inertial and inertialess contributions are naturally separated. The methodology for computation of charge transfer reorganization energies is developed and applied to a model two-site dipolar system in the SPC water solvent. Three types of charge transfer reactions are considered. The standard linear-response approach yields high accuracy for each particular reaction, but proves to be significantly in error when reorganization energies of different reactions were compared. This result has a purely molecular origin and is absent within a conventional continuum solvent model.

Polarizable Mean-Field Model of Water for Biological Simulations with AMBER and CHARMM Force Fields

“…However, reasonable value for a model of point charges should be close to $1/\sqrt{{\varepsilon }_{\text{italicel}}}$, i.e. about 0.75 (for water ε el =1.78), which results in the value of effective dipole 16-18 μ eff ≃ 2.35 D .…”

“…The first two terms do not explicitly contribute to intermolecular forces $F_i=-\frac{\partial U}{\partial r_i}$; however, they need to be included in the potential energy, enthalpy and especially in solvation energy calculations 15,17-18,28-30 . The appearance in theory of these new terms also suggests a new approach for the parameterization of bonded potential.…”

Activation entropy of electron transfer reactions

“…In particular, the dielectric response is very slowly converging in simulations and is potentially affected by approximations made to describe the long-range electrostatic forces. Several simulation protocols in which polarization response is (partially) integrated out by analytical techniques have been proposed [20,21]. Integral equation theories have been successfully applied to small solutes [22], but examples of their application to solvation and reactivity of large solutes are just a few [23].…”

Polarizable molecular interactions in condensed phase and their equivalent nonpolarizable models

“…20 The relations considered above demonstrate the essence of the approach: in condensed state, all polarization effects are reduced to some sort of scaling of the original fixed charges and dipoles of the model. The charge scaling has been discussed earlier on the basis of much simpler and pure intuitive picture [30,31,36]. The present theory provides additional insights and rigor into the question of how the screening factors D() should be selected.…”

“…The theory states that if simulations involve only similar configurations so that there is a well-defined average molecular dipole moment or, more generally, a welldefined average charge distribution within the molecule, then a typical set of fixed 30 parameters, such as charges, can be introduced that represent an averaged charge distribution [37]. We showed that the remaining fluctuations of charges around the average can be included as an additional renormalization or uniform scaling of the meanfield parameters [31,32].…”