PyMOL builds usually include a copy of a plugin for APBS (Adaptive Poisson-Boltzmann Solver). This plugin was written by Michael George Lerner and APBS itself is an open-source project run by Nathan A. Baker.

Currently, this plugin is not officially supported, though it is expected to become a supported feature during the lifetime of the 1.x release series.

More Information about APBS can be found on the PyMOL Wiki.