# Command to calculate energy of newly defined groups from an pre calculated xtc file **URL:** \ **Category:** User discussions\ **Tags:** mdp-parameters, mdrun\ **Created:** [August 24, 2022, 8:28am UTC](https://gromacs.bioexcel.eu/t/command-to-calculate-energy-of-newly-defined-groups-from-an-pre-calculated-xtc-file/4668 "2022-08-24T08:28:22Z")\ **Posts on this page:** 2\ **Page:** 1
**Author:** ![samapika](https://avatars.discourse-cdn.com/v4/letter/s/9fc348/32.png) [@samapika](https://gromacs.bioexcel.eu/u/samapika)\ **Post date:** [August 24, 2022, 8:28am UTC](https://gromacs.bioexcel.eu/t/command-to-calculate-energy-of-newly-defined-groups-from-an-pre-calculated-xtc-file/4668/1 "2022-08-24T08:28:22Z")
GROMACS version: 2018.1 GROMACS modification: No I have already completed the simulation. But now I am interested in calculating the energy of different system subgroups. I found that I can evaluate the same using gmx mdrun -rerun command. Do I need to generate a fresh tpr file and feed in the command? How should I extract the new edr file containing the energy of the new subgroups? Please suggest. ---
**Author:** ![alevilla](https://dub1.discourse-cdn.com/flex017/user_avatar/gromacs.bioexcel.eu/alevilla/32/439_2.png) [@alevilla](https://gromacs.bioexcel.eu/u/alevilla)\ **Post date:** [August 24, 2022, 9:53am UTC](https://gromacs.bioexcel.eu/t/command-to-calculate-energy-of-newly-defined-groups-from-an-pre-calculated-xtc-file/4668/2 "2022-08-24T09:53:57Z")
Hi, this post may already answer to one of your questions [Calculating vdW and ES energy between molecules](https://gromacs.bioexcel.eu/t/calculating-vdw-and-es-energy-between-molecules/386) > [@samapika](#): > > How should I extract the new edr file containing the energy of the new subgroups? You can use gmx energy to extract energy from energy file \*.edr. \Alessandra