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GROMACS modification: Yes/No
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Can I follow following link to calculate gmx_MMPBSA for charmm force field?

I follow the following command but got a positive free energy? As far as my experience this is totally wrong. What may be the error/reason?
gmx_MMPBSA -O -i -cs com.tpr -ci index.ndx -cg 1 13 -ct com_traj.xtc -cp

Your help is highly appreciated!