I calculated the coulombic surface and the electrostatic surface for my molecule. They show a significant difference in the shape of a small surface area. For the electrostatic surface, I converted the pdb file into a pqr file using the pdb2pqr called from chimera. Then I ran APBS also from within Chimera. When I display this surface it exhibits a strong protrusion in an area where there is no side chain. This protrusion is not present in the Coulombic surface. Any idea where this is coming from?