# Fix coordinates of specific residues for simulation

**URL:** <https://gromacs.bioexcel.eu/t/fix-coordinates-of-specific-residues-for-simulation/7039>\
**Category:** User discussions\
**Created:** [August 18, 2023, 6:48am UTC](https://gromacs.bioexcel.eu/t/fix-coordinates-of-specific-residues-for-simulation/7039 "2023-08-18T06:48:07Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![Anvithaa](https://avatars.discourse-cdn.com/v4/letter/a/59ef9b/32.png) [@Anvithaa](https://gromacs.bioexcel.eu/u/Anvithaa)\
**Post date:** [August 18, 2023, 6:48am UTC](https://gromacs.bioexcel.eu/t/fix-coordinates-of-specific-residues-for-simulation/7039/1 "2023-08-18T06:48:07Z")

</div>

GROMACS version:2021  
GROMACS modification: Yes/No  
How to apply positional restrain for specific residues in the protein that those coordinates remain fixed when Production run is performed?

---

<div class="post-metadata">

**Author:** ![Karis](https://avatars.discourse-cdn.com/v4/letter/k/ed8c4c/32.png) [@Karis](https://gromacs.bioexcel.eu/u/Karis)\
**Post date:** [August 18, 2023, 7:13am UTC](https://gromacs.bioexcel.eu/t/fix-coordinates-of-specific-residues-for-simulation/7039/2 "2023-08-18T07:13:06Z")

</div>

If you’re generating the files with pdb2gmx as your first step, it should generate a position restraint file (posre.itp). You can edit this file to only include those atoms you want to constrain, and as long as the topology file includes it under a #include line and the mdp file calls it using the define = -DPOSRES line. The position restraint file specifies atoms based on the atom number, the topol.top file should tell you which atoms were assigned which numbers