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Re: [lammps-users] compute stress/atom command
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Re: [lammps-users] compute stress/atom command

From: Axel Kohlmeyer <akohlmey@...24...>
Date: Fri, 10 Nov 2017 14:15:12 -0500

On Fri, Nov 10, 2017 at 1:36 PM, ruiyan <r-yan14@...61...> wrote:
Hi all, 

I try to use the compute stress/atom command. Its syntax is:

compute ID group-ID stress/atom temp-ID keyword  ...

In the manual, it's said that the temp-ID argument can be used to affect the per-atom velocities used in the kinetic energy contribution to the total stress. If the kinetic energy is *not included* in the stress, than the temperature compute is not used and can be specified as *NULL*. If the kinetic energy *is included* and you wish to use atom velocities as-is, then temp-ID can also be specified as *NULL*. 

I'm confused by this paragraph. From this paragraph, I don't know whether per-atom velocities are used when I set temp-ID to be NULL in this command.

​whether you use NULL or not has no impact on whether velocities are included or not. that is selected by the additional keywords or lack thereof.

by default (= no keyword), *all* components are included, and if you set NULL, it will use the standard velocites as defined by the standard temperature compute. it is irrelevant whether you restrict the stress computation to a specific group or not. as is *explicitly* explained in the manual, the definition of a temperature compute can be used to *subtract* a bias before including the velocities, e.g. in a flowing medium.​

In my simulation, group-ID is not 'all', and I want to include the kinetic energy in the compute, i.e. use the per-atom velocities in the specified group, should I use NULL or thermo_temp as temp-ID in this command? 

​using NULL or thermo_temp would be the same.​

please re-read the page in the manual and you will see.





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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.