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Re: [lammps-users] group/group force
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Re: [lammps-users] group/group force

From: Axel Kohlmeyer <akohlmey@...24...>
Date: Thu, 12 Oct 2017 01:19:49 -0400

On Wed, Oct 11, 2017 at 1:06 PM, <> wrote:
Dear all
There is a box which has 60000 gold atoms with fcc crystal structure 
Now I want to divide it in to groups which each group has 8 atoms then I should calculate interactive forces between them 
Is there any command for it ?

​not really.​
I have tried group command but lammps accept just 32 groups

​any more elaborate subdivision of the system should be done using the "chunk feature" in LAMMPS via compute atom/chunk.
assuming that you use an EAM style potential, a per-pair of chunks force tally for pair style​ could be programmed (in c++) using the ev_tally hook system as used by other /tally computes in the USER-TALLY package. in this case, you would have to map each atom in a pair of atoms to its chunk id and then tally the forces into a chunk-id vs. chunk-id triangular matrix and then process them, e.g. average them or write to a file for post-processing.

as mentioned before, this is a futile undertaking unless you have learned more about how to properly parameterize pairwise potentials, and having chosen which of the published methods for building (or guessing?) a coarse grained model you want to use.


Thanks for your help

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Dr. Axel Kohlmeyer  akohlmey@...12...24...
College of Science & Technology, Temple University, Philadelphia PA, USA
International Centre for Theoretical Physics, Trieste. Italy.