I have run a simulation of protein-ligand complex with Gromacs-5.1.4 and tried to calculated the MMPBSA using g_mmpbsa. I found that the total binding energy was -1750 kJ/mol and electrostatic energy was also too high. The system has a charge of -17. I tried different methods by using PBSolver=npbe, cfac=3, fadd=30 and with solute dielectric constant pdie=8 and pdie=20 still the energy values are too high.
Can anyone help me how to solve this problem?