# How to use Buckingham function

**URL:** <https://gromacs.bioexcel.eu/t/how-to-use-buckingham-function/1181>\
**Category:** User discussions\
**Created:** [December 2, 2020, 8:08am UTC](https://gromacs.bioexcel.eu/t/how-to-use-buckingham-function/1181 "2020-12-02T08:08:45Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![liyuqing](https://avatars.discourse-cdn.com/v4/letter/l/90db22/32.png) [@liyuqing](https://gromacs.bioexcel.eu/u/liyuqing)\
**Post date:** [December 2, 2020, 8:08am UTC](https://gromacs.bioexcel.eu/t/how-to-use-buckingham-function/1181/1 "2020-12-02T08:08:45Z")

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GROMACS version: 2016  
GROMACS modification: No  
I was troubled with how to used the Buckingham function for nonbonded interaction. As we all know ,the LJ has two parameters such as epsilon and sigema ,however the Buckingham has three parameters A\B\C.I don’t konw how to apply the Buckingham function or just revision the Buckingham function to L-J function…Some workers had said that to use the table potential.I have learn the manuel.but do not know how to make the systerm to invoke the table.xvg .

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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:** [December 4, 2020, 4:07pm UTC](https://gromacs.bioexcel.eu/t/how-to-use-buckingham-function/1181/2 "2020-12-04T16:07:33Z")

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Hi,  
Buckingham potential for non bonded interaction is implemented in GROMACS see:

> **[Non-bonded interactions — GROMACS 2020.4 documentation](http://manual.gromacs.org/current/reference-manual/functions/nonbonded-interactions.html)**

Table 13 in the link below show you the syntax that you have to use to define such a potential in the topology files

> **[File formats — GROMACS 2020.4 documentation](http://manual.gromacs.org/current/reference-manual/topologies/topology-file-formats.html?highlight=force%20field%20table)**

best regards  
Alessandra

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<div class="post-metadata">

**Author:** ![liyuqing](https://avatars.discourse-cdn.com/v4/letter/l/90db22/32.png) [@liyuqing](https://gromacs.bioexcel.eu/u/liyuqing)\
**Post date:** [December 8, 2020, 3:09am UTC](https://gromacs.bioexcel.eu/t/how-to-use-buckingham-function/1181/3 "2020-12-08T03:09:20Z")

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I am really appreciate for your reply, and I have followed the manual to define the potential. but it doesn’t work . I have tried with the OPLSAA forcefield, and here is the steps :  
1、set the parameters of the forcefiled.itp file at the defualt part  
[defaults]  
；nbfunc comb-rule gen-pairs fudgeLJ fudgeQQ  
2 1 no 0.5 0.5  
2、then at the same root bulit the table.xvg files of all the atom pairs.used the parameter B of buckingham to produce the three function combined to the table.xvg files. here was seven colum datas of r, f®, −f’®, g®,−g’®, h® and −h’® ,and r 's value range was[0,r\_cut off +1]  
3、went to the ffnonbonded.itp files to write the AC parameter of buckingham in form of epsilon and sigema.  
But Gromacs send the error that:Generating pair parameters is only support with L-J nonbonded function.  
I really puzzled with that . it means that this forcefield just appropriate to L-J nonbonded interaction?  
thank you very much !

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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:** [December 10, 2020, 8:40am UTC](https://gromacs.bioexcel.eu/t/how-to-use-buckingham-function/1181/4 "2020-12-10T08:40:53Z")

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Hi,  
The combination rules option (in forcefield.itp) is only for L-J parameters.  
When you use a Buckingham potential (func 2) you should define all the pairs in the directive [nonbond\_params] in ffnonbonded.itp. To generate such parameters you have to use the Buckingham combination rule given here [http://manual.gromacs.org/current/reference-manual/topologies/parameter-files.html#nbpar](http://manual.gromacs.org/current/reference-manual/topologies/parameter-files.html#nbpar)  
This should resolve your error message

Finally, I do not follow why you generate tabulated potentials. If a functional form is already implemented, you do not need to generate tabulate potential. But maybe I am missing something.

best regards  
Alessandra