where ρ(r) is the molecule's continuous electronic density, ZA is the charge on nucleus A at RA, and N is the total number of nuclei in the molecular system. The first term in Equation ( ) is the bare nuclear potential (BNP), and the second term represents the electronic contribution. The electric potential at a point in space is defined as the work done by an external force in bringing a positive test charge from infinity to that point, divided by the magnitude of the test charge . MESP analysis acts as a map of attraction and repulsion for protons within a molecule. Areas with negative electrostatic potential, rich in electron density (lone pairs, π-bonds), The electron density isosurface represents a geometric locus within the molecule where the electron density assumes a specific predetermined value.