Implicit solvation model

CASTEP offers a minimal-parameter implicit solvation model. This is based on the iso-density scheme first proposed in a series of papers (Fattebert and Gygi (2002), Fattebert and Gygi (2003), Scherlis et al. (2006)). Dziedzic made refinements to the scheme, see Dziedzic et al. (2011).

DFT describes the solute, while a dielectric continuum with a spatially varying permittivity represents the solvent. Unlike most traditional solvation models, here the permittivity changes smoothly, approaching the bulk value far from the solute, and 1 in the vicinity of the solute. This is achieved by relating the permittivity to the electronic density of the solute. In this way, definition of the solute cavity is natural, avoiding the need for parameterizing its shape – the model is free of the ionic radii needed in most models.

This requires only two parameters to define the model; the density where the permittivity drops to half the bulk value and the steepness of this transition. With suitable, transferable values determined in advance. These are independent of solute, solvent, and details of the DFT calculation, such as pseudopotentials or exchange-correlation functionals.

Two physical characteristics describe the solvent itself: bulk permittivity (dielectric constant) and surface tension.

Apart from electrostatic effects of the solvent, the CASTEP model includes the apolar contribution to solvation-cavitation energy, and solute-solvent dispersion-repulsion energy. The SASA (solvent-accessible surface area) approximation calculates both of these terms, so they are proportional to the surface area of the cavity.

Dziedzic et al. (2011) includes a brief description of the model, discussion of its implementation, and comparison to other approaches. Dziedzic et al. (2013) provides a more detailed description of the choice of numerical parameters. Fox et al. (2014) presents an example application to solvation of an entire protein.

See Also:

Theory in CASTEP