[lammps-users] compute ID group-ID stress/atom temp-ID

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1

Hello LAMMPS users,

I am using the LAMMPS window version 19 March 2020. In lammps doc, it is mentioned
“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 am using below commands:

(1)
compute sigmaalla all stress/atom NULL # gives virial plus KE
compute sigmaall all reduce sum c_sigmaalla[1] c_sigmaalla[2] c_sigmaalla[3]

(1) means NULL means KE takes thermal velocity + particle velocity or advection velocity?

(2)
compute sigmaallatem all stress/atom thermo_temp # gives virial plus KE
compute sigmaalltem all reduce sum c_sigmaallatem[1] c_sigmaallatem[2] c_sigmaallatem[3]

(2) means thermo_temp means KE takes thermal velocity only not particle velocity?

I am doing tensile test on a graphene using the deform command. I am getting the same results for either use NULL or thermo_temp?
sigmaall[1] = sigmaalltem[1]
sigmaall[2] = sigmaalltem[2]

image.png

Is it because particle velocity contribution in case (1) is very less? Should I consider particle velocity while calculating stress?

Thanks and regards
Ankit

2

(1) What are those terms: thermal velocity, particle velocity, advection velocity?

(2) If there are no changes to the default settings in your input, the output from
compute all stress/atom NULL
and
compute all stress/atom thermo_temp
should be identical. That is what the documentation says and that is what I just confirmed using the 20 September 2021 version of LAMMPS.

image.png