# gmx\_MMPBSA

**URL:** <https://gromacs.bioexcel.eu/t/gmx-mmpbsa/3986>\
**Category:** User discussions\
**Created:** [April 8, 2022, 4:20pm UTC](https://gromacs.bioexcel.eu/t/gmx-mmpbsa/3986 "2022-04-08T16:20:31Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![Santosh456](https://avatars.discourse-cdn.com/v4/letter/s/a8b319/32.png) [@Santosh456](https://gromacs.bioexcel.eu/u/Santosh456)\
**Post date:** [April 8, 2022, 4:20pm UTC](https://gromacs.bioexcel.eu/t/gmx-mmpbsa/3986/1 "2022-04-08T16:20:31Z")

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GROMACS version:  
GROMACS modification: Yes/No  
Here post your question  
Can I follow following link to calculate gmx\_MMPBSA for charmm force field?

> **[Protein-ligand (Charmm) - gmx\_MMPBSA Documentation](https://valdes-tresanco-ms.github.io/gmx_MMPBSA/examples/Protein_ligand_CHARMMff/)**
>
> gmx\_MMPBSA is a new tool based on AMBER's MMPBSA.py aiming to perform end-state free energy calculations with GROMACS files

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 mmpbsa.in -cs com.tpr -ci index.ndx -cg 1 13 -ct com\_traj.xtc -cp topol.top

Your help is highly appreciated!