Umbrella Sampling Negative Potential Mean Force for Protein-Ligand System

GROMACS version: 2018.1
GROMACS modification: Yes/No
Here post your question
profile.xvg (14.0 KB)
The final value of the potential mean force for this system is negative. How can I interpret this diagram? Or is there something wrong in the way I am handling this system?

I would think this PMF is exactly what one would expect: a negative free-energy at short distance for a ligand binding to a protein.