The electrostatic potential method used in MOPAC is severely out-of-date. The use of ESP is strongly discouraged. A much faster and easier method is to use keyword GRAPHF.
This is the ElectroStatic Potential calculation of K. M. Merz and B. H. Besler [31]. ESP calculates the expectation values of the electrostatic potential of a molecule on a uniform distribution of points. The resultant ESP surface is then fitted to atom centered charges that best reproduce the distribution, in a least squares sense. To print out the ESP map, use POTWRT.