Your question is not clear, but I think you mean how to calculate the electrostatic potential at a certain coordinate R originating from certain atom r_i. If this is the case, you must take the vector difference (R - r_i), then calculate the length of that vector, call that d_i. The electrostatic potential is (1/(4 Pi e0))*(q_i/d_i). Where e0 is the dielectric constant, and q_i is the point charge assigned to atom i by GROMACS. Then you must add over all atoms you want (sum over index i).

Hello.

You can create an index file (index.ndx) with a group containing the atom you are interessed and use the command g_potential. Check the manual: http://manual.gromacs.org/online/g_potential.html

Best regards,

Ricardo Ferreira