# Buckingham + 18-6 Lennard-Jones

**URL:** <https://gromacs.bioexcel.eu/t/buckingham-18-6-lennard-jones/2707>\
**Category:** User discussions\
**Tags:** forcefield, grompp\
**Created:** [August 20, 2021, 3:15pm UTC](https://gromacs.bioexcel.eu/t/buckingham-18-6-lennard-jones/2707 "2021-08-20T15:15:14Z")\
**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:** [August 20, 2021, 3:15pm UTC](https://gromacs.bioexcel.eu/t/buckingham-18-6-lennard-jones/2707/1 "2021-08-20T15:15:14Z")

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GROMACS version: 2021  
GROMACS modification: No

Hi,

I would like to try some annealing on amorphos silica; in order to do so I either need a force field with triplets potential (which I rekon would be quite inefficient) or a pair potential that combines Buckingham and 18-6 Lennard-Jones.

Is it possible to do the latter in GROMACS, without recurring to tabulated potentials and/or to a 12-6 LJ approximation? I see I can define Buckingham parameters and combination rules, but I am not sure if (a) I can combine it with Lennard-Jones and (b) if I can define arbitrary powers for the repulsive part of LJ.

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**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:** [August 25, 2021, 9:45am UTC](https://gromacs.bioexcel.eu/t/buckingham-18-6-lennard-jones/2707/2 "2021-08-25T09:45:23Z")

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Hi,  
The answer is to both question is no. The alternative is to use tabulated potential, but you have to down-grade to version 2018.  
The tabulated potentials are not active from 2019 and they will probably come back in the future.

Kind regards  
Alessandra

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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:** [August 25, 2021, 10:04am UTC](https://gromacs.bioexcel.eu/t/buckingham-18-6-lennard-jones/2707/3 "2021-08-25T10:04:10Z")

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Hi,

I will try with an approximation then

Thanks