# Repulsive potential between virtual sites

**URL:** <https://gromacs.bioexcel.eu/t/repulsive-potential-between-virtual-sites/3672>\
**Category:** User discussions\
**Created:** [February 15, 2022, 8:02am UTC](https://gromacs.bioexcel.eu/t/repulsive-potential-between-virtual-sites/3672 "2022-02-15T08:02:26Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![rainbow](https://avatars.discourse-cdn.com/v4/letter/r/7993a0/32.png) [@rainbow](https://gromacs.bioexcel.eu/u/rainbow)\
**Post date:** [February 15, 2022, 8:02am UTC](https://gromacs.bioexcel.eu/t/repulsive-potential-between-virtual-sites/3672/1 "2022-02-15T08:02:26Z")

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GROMACS version: 2021.4  
GROMACS modification: No

Dear gmx users,

System : Protein in water

In charmm36 and amber99sb-ildn, comb-rule = 2.

When setting up a repulsive potential between virtual sites, can I apply the same conversion of Rmin (1.2 nm) to sigma (1.06908) for both force fields,? Or, any other conversions required?

Thanks for your help.

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<div class="post-metadata">

**Author:** ![hess](https://dub1.discourse-cdn.com/flex017/user_avatar/gromacs.bioexcel.eu/hess/32/416_2.png) [@hess](https://gromacs.bioexcel.eu/u/hess)\
**Post date:** [February 21, 2022, 1:49pm UTC](https://gromacs.bioexcel.eu/t/repulsive-potential-between-virtual-sites/3672/2 "2022-02-21T13:49:25Z")

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The Rmin to sigma coversion is general for the Lennard-Jones potential and does not depend on the force field. But there might be unit conversions that depend on the force fields you convert between.