Figure 1.

The TK model. The TK model equates the solvent-excluded volume of a solute (yellow) with a sphere of low dielectric material. Atoms in the solute (black and gray circles) are mapped to corresponding locations in the sphere. Electrostatic energies can be calculated rapidly in the simplified spherical geometry. Fractional solvent-accessible surface areas are determined for a series of concentric probe shells (dashed lines) centered on each charged atom of the solute (black circles). Charged atoms are mapped into the sphere such that a weighted average of the probe-shell accessible surface area is identical in the solute and sphere geometries.