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Re: [lammps-users] fix rigid/npt with triclinic
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Re: [lammps-users] fix rigid/npt with triclinic

From: Axel Kohlmeyer <akohlmey@...24...>
Date: Mon, 28 Aug 2017 16:02:03 -0400

On Mon, Aug 28, 2017 at 3:32 PM, Wes Barnett <w.barnett@...6288.....> wrote:

​it is a case of "better safe than sorry". you can have only one fix changing the cell shape and volume, and that has to be coupled to the complete system pressure. that is usually for group all, i.e. what the thermo press keyword outputs. that should be done correctly by the npt or nph fix, but there is no harm in making it explicit.

Interestingly with iso or aniso pressure I am able to have more than one fix npt without any errors or warnings. If I do:

fix 1 rigidGroup rigid/nvt molecule temp 1.0 1.0 0.5
fix 2 nonRigidGroup npt temp 1.0 1.0 0.5 iso 5.0 5.0 5.0

Is that the same as the following practically speaking?

fix 1 rigidGroup rigid/npt molecule temp 1.0 1.0 0.5 iso 5.0 5.0 5.0
fix 2 nonRigidGroup npt temp 1.0 1.0 0.5 iso 5.0 5.0 5.0


The manual says that I "cannot" use both "fix npt" and "fix rigid/npt" since only one is coupled to system pressure, so I would have expected an error message. Is the last example above supported by LAMMPS or should it be avoided? 

​there is currently no simple test for that. the problem is, that there are cases, where you would want, say, fix deform coupled to one dimension and fix npt to the other two dimensions, which should be allowed, even though there are two different fixes trying to modify the system box.

an invalid setup should reveal itself quickly by producing a bogus trajectory or even a crash.



James "Wes" Barnett
Postdoctoral Research Scientist
Department of Chemical Engineering

Dr. Axel Kohlmeyer  akohlmey@...92......
College of Science & Technology, Temple University, Philadelphia PA, USA
International Centre for Theoretical Physics, Trieste. Italy.