# How to specify C6/C12 instead of sigma/epsilon in topologies?

**URL:** <https://gromacs.bioexcel.eu/t/how-to-specify-c6-c12-instead-of-sigma-epsilon-in-topologies/3084>\
**Category:** User discussions\
**Created:** [November 5, 2021, 2:46pm UTC](https://gromacs.bioexcel.eu/t/how-to-specify-c6-c12-instead-of-sigma-epsilon-in-topologies/3084 "2021-11-05T14:46:43Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![MichelePellegrino](https://dub1.discourse-cdn.com/flex017/user_avatar/gromacs.bioexcel.eu/michelepellegrino/32/628_2.png) [@MichelePellegrino](https://gromacs.bioexcel.eu/u/MichelePellegrino)\
**Post date:** [November 5, 2021, 2:46pm UTC](https://gromacs.bioexcel.eu/t/how-to-specify-c6-c12-instead-of-sigma-epsilon-in-topologies/3084/1 "2021-11-05T14:46:43Z")

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GROMACS version: 2021  
GROMACS modification: No

Hi,

I would like to simulate a silica substrate having a complex 6-18+Buckingham potential. I managed to find what I believe is a decent approximation using only LJ with expressions:

C12_r^-12 - C6_r^-6

However, my C6/C12 constant are not convertible into sigma/epsilon (e.g. for a couple of atom types C12=0).

I was wondering if there is a way to specify C12/C6 instead of sigma/epsilon when specify the nonbonded parameters in `.itp` files.

p.s. I am assuming GROMACS can handle C6/C12 LJ potentials, but I might be wrong

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**Author:** ![pjohansson](https://dub1.discourse-cdn.com/flex017/user_avatar/gromacs.bioexcel.eu/pjohansson/32/316_2.png) [@pjohansson](https://gromacs.bioexcel.eu/u/pjohansson)\
**Post date:** [November 5, 2021, 2:55pm UTC](https://gromacs.bioexcel.eu/t/how-to-specify-c6-c12-instead-of-sigma-epsilon-in-topologies/3084/2 "2021-11-05T14:55:29Z")

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Hi,

as I understand the manual this is done by setting combination rule to 1 in your `[defaults]` directive.

Link: [Parameter files — GROMACS 2021.4 documentation](https://manual.gromacs.org/current/reference-manual/topologies/parameter-files.html#non-bonded-parameters)

Note that this affects all interactions, so you’ll need to include your water model files etc. explicitly and change their values to equivalent C6/12.

Petter

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<div class="post-metadata">

**Author:** ![MichelePellegrino](https://dub1.discourse-cdn.com/flex017/user_avatar/gromacs.bioexcel.eu/michelepellegrino/32/628_2.png) [@MichelePellegrino](https://gromacs.bioexcel.eu/u/MichelePellegrino)\
**Post date:** [November 5, 2021, 3:02pm UTC](https://gromacs.bioexcel.eu/t/how-to-specify-c6-c12-instead-of-sigma-epsilon-in-topologies/3084/3 "2021-11-05T15:02:18Z")

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Ah, I see. Thanks Petter.

It’s a bit counter-intuitive tough, since you would think that the _combination_ rule only affects the… _combination_ between differente atomtypes, not the way parameters are specified for each atomtype.

I don’t have any water at this point in the system, I am just trying to simulate the annealing of a silica slab; I will use another force field later when introducing water too.