title: Free energy functionals for polarization fluctuations: Pekar factor revisited.
abstract: The separation of slow nuclear and fast electronic polarization in problems related to electron mobility in polarizable media was considered by Pekar 70 years ago. Within dielectric continuum models, this separation leads to the Pekar factor in the free energy of solvation by the nuclear degrees of freedom. The main qualitative prediction of Pekar's perspective is a significant, by about a factor of two, drop of the nuclear solvation free energy compared to the total (electronic plus nuclear) free energy of solvation. The Pekar factor enters the solvent reorganization energy of electron transfer reactions and is a significant mechanistic parameter accounting for the solvent effect on electron transfer. Here, we study the separation of the fast and slow polarization modes in polar molecular liquids (polarizable dipolar liquids and polarizable water force fields) without relying on the continuum approximation. We derive the nonlocal free energy functional and use atomistic numerical simulations to obtain nonlocal, reciprocal space electronic and nuclear susceptibilities. A consistent transition to the continuum limit is introduced by extrapolating the results of finite-size numerical simulation to zero wavevector. The continuum nuclear susceptibility extracted from the simulations is numerically close to the Pekar factor. However, we derive a new functionality involving the static and high-frequency dielectric constants. The main distinction of our approach from the traditional theories is found in the solvation free energy due to the nuclear polarization: the anticipated significant drop of its magnitude with increasing liquid polarizability does not occur. The reorganization energy of electron transfer is either nearly constant with increasing the solvent polarizability and the corresponding high-frequency dielectric constant (polarizable dipolar liquids) or actually noticeably increases (polarizable force fields of water).
authors.affiliation: School of Molecular Sciences, Arizona State University, P.O. Box 871604, Tempe, Arizona 85287-1604, USA.
authors.forename: Mohammadhasan
authors.identifier: 0000000215470334
authors.initials: M
authors.lastname: Dinpajooh
authors.affiliation: Chemistry Department, Brookhaven National Laboratory, P.O. Box 5000, Upton, New York 11973-5000, USA.
authors.forename: Marshall D
authors.initials: MD
authors.lastname: Newton
authors.affiliation: Department of Physics and School of Molecular Sciences, Arizona State University, P.O. Box 871504, Tempe, Arizona 85287-1504, USA.
authors.forename: Dmitry V
authors.identifier: 000000029352764X
authors.initials: DV
authors.lastname: Matyushov