# Conversion of OPLS ITP file to charmm/gromacs itp file

**URL:** <https://gromacs.bioexcel.eu/t/conversion-of-opls-itp-file-to-charmm-gromacs-itp-file/1641>\
**Category:** User discussions\
**Created:** [February 25, 2021, 9:18am UTC](https://gromacs.bioexcel.eu/t/conversion-of-opls-itp-file-to-charmm-gromacs-itp-file/1641 "2021-02-25T09:18:09Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![Yogesh](https://avatars.discourse-cdn.com/v4/letter/y/49beb7/32.png) [@Yogesh](https://gromacs.bioexcel.eu/u/Yogesh)\
**Post date:** [February 25, 2021, 9:18am UTC](https://gromacs.bioexcel.eu/t/conversion-of-opls-itp-file-to-charmm-gromacs-itp-file/1641/1 "2021-02-25T09:18:09Z")

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greetings users,

I have generated an gromacs compatible ff for a ligand from LIGPargen.  
ITP is in OPLS ff format having sections as follows.  
**; GENERATED BY LigParGen Server**  
**;**  
**[atomtypes]**  
**[moleculetype]**  
**[atoms]**  
**[bonds]**  
**[angles]**  
**[dihedrals]**  
\*\*; IMPROPER DIHEDRAL ANGLES \*\*  
**[dihedrals]**  
**; PROPER DIHEDRAL ANGLES**  
**[pairs]**

I want to convert the parameters to gromacs compatible charmm FF. this FF require sections like

**CHARMM36 FF in GROMACS format**

**[defaults]**  
**[atomtypes]**  
**; name at.num mass charge ptype sigma epsilon ; sigma\_14 epsilon\_14**  
\*\* [bondtypes]\*\*  
**[angletypes]**  
**; i j k func th0 cth S0 Kub**  
**[dihedraltypes]**  
**; i j k l func phi0 cp mult**  
can someone suggest way to interconvert the parameters?  
I will be really thankful for your help.

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<div class="post-metadata">

**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:** [February 25, 2021, 10:01am UTC](https://gromacs.bioexcel.eu/t/conversion-of-opls-itp-file-to-charmm-gromacs-itp-file/1641/2 "2021-02-25T10:01:14Z")

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Hi,  
just a general comment. Parameters compatible with one force field can not be used with another force fields. One can not mix force fields.  
Best regards  
Alessandra

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<div class="post-metadata">

**Author:** ![Yogesh](https://avatars.discourse-cdn.com/v4/letter/y/49beb7/32.png) [@Yogesh](https://gromacs.bioexcel.eu/u/Yogesh)\
**Post date:** [February 25, 2021, 10:58am UTC](https://gromacs.bioexcel.eu/t/conversion-of-opls-itp-file-to-charmm-gromacs-itp-file/1641/3 "2021-02-25T10:58:23Z")

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Dr. alevilla .  
thankyou for your suggestion. iam dealing with a rare molecule and it had been a challenge getting forcefields. I was thinking of converting LJ parameters only.

or the other way around, can you please post some server from where i can get POPC bilayer parameters in OPLSAA forcefield?  
that would be great.