$esp_fitfits point charges at the positions of nuclei to electrostatic potential arising from electric charge distribution (for UHF cases also for spin density, also possible in combination with $soghf). For this purpose the ("real") electrostatic potential is calculated at spherical shells of grid points around the atoms. By default, Bragg-Slater radii, rBS, are taken as shell radii.
A parametrization very close to that suggested by Kollman (a multiple-shell model with shells of radii ranging from 1.4*rvdW to 2.0*rvdW, rvdW is the van-der-Waals radius; U.C. Singh, P.A. Kollman, J. Comput. Chem. 5(2), 129-145 (1984)) is used if the keyword is extended: