# Define ATP+mg in Amber forcefield

**URL:** <https://gromacs.bioexcel.eu/t/define-atp-mg-in-amber-forcefield/13237>\
**Category:** User discussions\
**Tags:** forcefield\
**Created:** [February 23, 2026, 7:21pm UTC](https://gromacs.bioexcel.eu/t/define-atp-mg-in-amber-forcefield/13237 "2026-02-23T19:21:55Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![Mobina](https://avatars.discourse-cdn.com/v4/letter/m/f04885/32.png) [@Mobina](https://gromacs.bioexcel.eu/u/Mobina)\
**Post date:** [February 23, 2026, 7:21pm UTC](https://gromacs.bioexcel.eu/t/define-atp-mg-in-amber-forcefield/13237/1 "2026-02-23T19:21:56Z")

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GROMACS version:2025  
GROMACS modification: No  
Hello everyone, I am using Amber99SB-ildn force field to simulate a kinase in complex with ATP and Mg. I generated the protein topology using pdb2gmx. Then created ATP topology with Ambertools and attached it to my topol.top file. I also attached ions.itp file of forcefield for Mg parameters. But after EM Mg interacts with the Adenosine ring instead of the phosphate group. I tried to apply position restraint for ATP+Mg, but it did not work.  
I am new to this field, and I am not sure if I did it correctly or not. I would appreciate any suggestions or guidance.  
I have to mention that I extracted ATP coordinates of my protein complex to generate topology, not the standard ATP coordinates from PDB.