# How to make oplsaa.ff .itp files for compounds with charge greater than 2

**URL:** <https://gromacs.bioexcel.eu/t/how-to-make-oplsaa-ff-itp-files-for-compounds-with-charge-greater-than-2/10244>\
**Category:** User discussions\
**Created:** [September 27, 2024, 5:34am UTC](https://gromacs.bioexcel.eu/t/how-to-make-oplsaa-ff-itp-files-for-compounds-with-charge-greater-than-2/10244 "2024-09-27T05:34:35Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![aish.warya\_2024](https://avatars.discourse-cdn.com/v4/letter/a/b5ac83/32.png) [@aish.warya\_2024](https://gromacs.bioexcel.eu/u/aish.warya_2024)\
**Post date:** [September 27, 2024, 5:34am UTC](https://gromacs.bioexcel.eu/t/how-to-make-oplsaa-ff-itp-files-for-compounds-with-charge-greater-than-2/10244/1 "2024-09-27T05:34:35Z")

</div>

GROMACS version:  
GROMACS modification: Yes/No  
Here post your question  
Dear users,

I have tried to generate an OPLSAA.ff .itp file for a polymer with different charges. Using LIGPARGEN, it is possible to go up to a charge of ±2. Are there any alternatives to generate a topology file for compounds with charges greater than ±2?

Thank you!

---

<div class="post-metadata">

**Author:** ![ahosseini9473](https://avatars.discourse-cdn.com/v4/letter/a/e274bd/32.png) [@ahosseini9473](https://gromacs.bioexcel.eu/u/ahosseini9473)\
**Post date:** [September 29, 2024, 12:58am UTC](https://gromacs.bioexcel.eu/t/how-to-make-oplsaa-ff-itp-files-for-compounds-with-charge-greater-than-2/10244/2 "2024-09-29T00:58:27Z")

</div>

Hi,  
Could you please mention what molecule?  
Basically, charges are dependent on a molecule you want to work on. Technically, you can modify charges if your project is in that direction.  
And finally, I would recommend building your ff.itp from scratch rather than using LigPargen. This generator has already issues with rounding charges.

---

<div class="post-metadata">

**Author:** ![aish.warya\_2024](https://avatars.discourse-cdn.com/v4/letter/a/b5ac83/32.png) [@aish.warya\_2024](https://gromacs.bioexcel.eu/u/aish.warya_2024)\
**Post date:** [October 3, 2024, 4:14am UTC](https://gromacs.bioexcel.eu/t/how-to-make-oplsaa-ff-itp-files-for-compounds-with-charge-greater-than-2/10244/3 "2024-10-03T04:14:02Z")

</div>

Thank you for your reply. I need to generate .itp file for polypyrrole contianing 4 repeating unit and charge +3. I am attaching the link of my pdb file

> **[py+3-4ru-opt65236.pdb](https://drive.google.com/file/d/1EXbAPV-0c6duXbSVqlkRpqOvuk0FxIM_/view?usp=sharing)**
>
> Google Drive file.

Thank you