Modeling Charge Distributions and Dielectric Response Functions of Atomistic and Continuous Media


Many physical processes involve a significant redistribution of charge density, be it in a central system of interest or in a polarisable embedding medium providing boundary conditions. Examples range from protein folding in an aqueous solvent to the charge transfer between two unlike solids in relative motion. As modelers, we wish to have at our disposal efficient methods allowing us to describe the relevant changes, for example, to predict in what way charge redistribution affects interatomic forces. At small scales, calculations can be based on density-functional theory, while continuum electrostatics is appropriate for the description at large scales. However, neither of the two methods is well-suited when space is discretised into volume elements of atomic dimensions. At that scale, the most intuitive description is in terms of partial charges plus potentially electrostatic dipoles or higher-order atomic multipoles. Their proper assignment is crucial when dealing with chemically heterogeneous systems, however, it turns out to be non-trivial. Particularly challenging is a description of the charge transfer between atoms. In this chapter, we discuss attempts to describe such charge distribution in the framework of force fields assigning partial charges with so-called charge equilibration methods. This includes their motivation from the bottom-up, i.e., through density functional theory. In the top-down design, we investigate how to construct the microscopic model so that it reproduces the desired macroscopic response to external fields or to an excess charge. Lastly, we present avenues to extend the atom-scale models to non-equilibrium situations allowing one to model contact electrification or the discharge of a Galvanic cell

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Last time updated on May 16, 2016

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