Dear Sergio,
If you're using PME, you need to also include the reciprocal contribution. Gerrit Dear gmx comunity, I am trying to calculate the electrostatic interaction of my system for force field development. My system consists of 7 positively charged particles interacting (system A) with another positively charged particle (system B). I calculate the electrostatic interaction by doing reruns of the trajectory in which some of them have been removed. E_int(electrostatic) = E_AB(coul-SR)+E_AB(coul-LR)-E_A(coul-SR)-E_A(coul-LR)-E_B(coul-SR)-E_B(coul-LR) And to my surprise I get negative energies. I know the PME method has its energy shifted: "With the Verlet cut-off scheme, the PME direct space potential is shifted by a constant such that the potential is zero at the cut-off. This shift is small and since the net system charge is close to zero, the total shift is very small, unlike in the case of the Lennard-Jones potential where all shifts add up. We apply the shift anyhow, such that the potential is the exact integral of the force." Since for the system with just one charge I get a possitive E_1q(coul-LR), I imagine the LR-coul term is shifted (eventhough I have to use the group cut-off scheme and not the verlet). The problem is why don't all these shifts cancel? Is there a way to not add the shifts? I leave bellow the .mdp parameters. Note that for the VDW part I need to use user-tables and therefore I need to use the group scheme. Thank you very much! Sergio P?rez-Conesa ; neighbour search cutoff-scheme = group nst-list = 10 verlet-buffer-tolerance = 0.005 ns-type = grid ; Pbc pbc = xyz ; xyz or xy ; Vdw rvdw = 1.4 rlist = 1.4 vdwtype = user ; PME or User to look for table /user DispCorr = No ; corrections to energy and pressure or No ;Electrostatic coulombtype = PME ; User if look for table rcoulomb = 1.4 pme-order = 4 fourierspacing = 0.2 ewald-geometry = 3d ; 3d or 3dc ;IMD- -- Gromacs Users mailing list * Please search the archive at http://www.gromacs.org/Support/Mailing_Lists/GMX-Users_List before posting! * Can't post? Read http://www.gromacs.org/Support/Mailing_Lists * For (un)subscribe requests visit https://maillist.sys.kth.se/mailman/listinfo/gromacs.org_gmx-users or send a mail to gmx-users-requ...@gromacs.org.