The GB is an effective approach to calculate charge-charge interactions $W_{ij}$ in the presence of continuum solvent:

$$W_{ij} = \underbrace{ {1\over 2} \sum_{i\not= j} {{q_i q_j}\over{... ...\sum_{i,j} \frac {{q_i q_j}} {f^{gb}(r_{ij})} }_{\Delta W,~~ solvation~ energy}$$ (1)

where ${\epsilon}_w$ - dielectric constant of the solvent (80), and $f^{gb}_{ij}$ is a certain smooth function which is assumed to depend only upon atomic radii ${\rho}_i$ and interatomic distances $r_{ij}$. A fairly simple analytic form of $f^{gb}_{ij}$ has been proposed by Still and shown to describe electrostatic interactions within small molecules quite well, as compared to the solution of Poisson-Boltzmann equation and experiment.