**Polarization as a field variable from molecular dynamics simulations**

KK Mandadapu and JA Templeton and JW Lee, JOURNAL OF CHEMICAL PHYSICS, 139, 054115 (2013).

DOI: 10.1063/1.4817004

A theoretical and computational framework for systematically calculating
the macroscopic polarization density as a field variable from molecular
dynamics simulations is presented. This is done by extending the
celebrated Irving and Kirkwood **J. Chem. Phys. 18, 817 (1950)**
procedure, which expresses macroscopic stresses and heat fluxes in terms
of the atomic variables, to the case of electrostatics. The resultant
macroscopic polarization density contains molecular dipole, quadrupole,
and higher-order moments, and can be calculated to a desired accuracy
depending on the degree of the coarse-graining function used to connect
the molecular and continuum scales. The theoretical and computational
framework is verified by recovering the dielectric constant of bulk
water. Finally, the theory is applied to calculate the spatial variation
of the polarization vector in the electrical double layer of a 1: 1
electrolyte solution. Here, an intermediate asymptotic length scale is
revealed in a specific region, which validates the application of mean
field Poisson-Boltzmann theory to describe this region. Also, using the
existence of this asymptotic length scale, the lengths of the diffuse
and condensed/Stern layers are identified accurately, demonstrating that
this framework may be used to characterize electrical double layers over
a wide range of concentrations of solutions and surface charges. (C)
2013 AIP Publishing LLC.

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