# Applying distance constraints to ligand and Mg ion **URL:** \ **Category:** User discussions\ **Created:** [July 20, 2020, 1:57pm UTC](https://gromacs.bioexcel.eu/t/applying-distance-constraints-to-ligand-and-mg-ion/540 "2020-07-20T13:57:11Z")\ **Posts on this page:** 1\ **Showing post:** 34
**Author:** ![Tanusha\_ghosh](https://avatars.discourse-cdn.com/v4/letter/t/b487fb/32.png) [@Tanusha\_ghosh](https://gromacs.bioexcel.eu/u/Tanusha_ghosh)\ **Post date:** [June 26, 2025, 12:45pm UTC](https://gromacs.bioexcel.eu/t/applying-distance-constraints-to-ligand-and-mg-ion/540/34 "2025-06-26T12:45:44Z")
Hi Josh. ## I am also facing a similar situation. My only confusion is that how you named the Mg and O of ligand as a\_10908 and a\_10900 respectively? It seems that you have your gro file to have chain a to be already defined, which I do not have in my case. FYI, I am attaching the link to my question in the grosmcs mailing forum: > [@Help with Pulling Simulation Between Two Atoms](https://gromacs.bioexcel.eu/t/help-with-pulling-simulation-between-two-atoms/12275): > > GROMACS version: 2025.2 GROMACS modification: No I’m working on a pulling simulation in GROMACS where I want to apply a force between atom N01 of a GUA residue and atom O05 of an ACE group. I’m trying to pull them apart by 1 Å over 0.5 ns, so I’ve set the pull rate to 0.0002 (2 Å/ns). I’m a bit confused about how to correctly define the pull\_group1\_name and pull\_group2\_name should I define them based on the residues (GUA and ACE) or create custom index groups for just N01 and O05 ? I’m not s… --- _[View the full topic](https://gromacs.bioexcel.eu/t/applying-distance-constraints-to-ligand-and-mg-ion/540)._