Nonbonded interactions are strongly distance-dependent: Coulomb potential, 1/r**2; repulsive term of Lennard-Jones potential, 1/r**12; attractive term of Lennard-Jones potential, 1/r**6.
Some programs also use 6.5 or 8.0 Angstroms as the maximum contact distance. This typically yields the first shell of residues around the probe.