Hi. I'm an undergraduate student volunteering in research and I'm trying to get some unsupervised work done while my instructor is on vacation. We are working with the correlations between cohesive energies as they relate to madelung constants in nanotubes. The basic gist (as far as I understand) is that there is a linear relationship with an r^2~1 for the tubes we have computed. I'm trying to calculate charges for the individual ions but the derived equation I was given doesn't seem to be accurate because other calculations which use calculated charges are way off. I'm looking around at other papers printed on this topic and I see that columbic energy=(MC*z*z)/r where MC=madelung constant, z is charge and r is distance. Another paper demonstrates that the slope (of cohesive energy vs. MC)=(-e^2*z^2)/r. What does the "e" stand for in the second equation? Is more info needed? thanks