What can be the variation of lennard jones long range interactions between enzyme and a metal ion?

GROMACS version:2018
GROMACS modification: Yes/No
I have md simulated an enzyme with a metal ion using CHARMM36 force feild. Using the trajectories I calculated the interaction energies Coul-SR, LJ-SR and LJ-14 between enzyme and the metal ion. However, for the Lennard Jones long range interactions I got zero energy through out the simulation. Is this possible?