[lammps-users] Minimum Strainrate used in Lammps

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1

Hello Lammps-users

I am performing a uniaxial tensile test on a calcium silicate hydrate structure of 150 atoms. I am using the buckingham and stillenger and weber potentials. The structure is minimized before strainrates are appplied. I am applying a constant strainrate in the x direction via the erate command and applying zero applied pressure to the other dimensions via fix npt anisto command. The stress is computed using the following command. Are these correct commands to compute the stress in the my system?

compute peratom all stress/atom
compute p all reduce sum c_peratom[1] c_peratom[2] c_peratom[3]
variable press equal -(c_p[1]+c_p[2]+c_p[3])/(3*vol)
thermo_style custom step temp etotal press v_press

I am currently using strainrates of 0.00002/ps and 0.002/ps. At these rates I get really low negative stresses (-3,000MPa) for tensile strains and high positive values for compressive strains. I even tried a strainrate of 2.96e-15/ps but the run was aborted.

Also, I wanted to know is there a minimum strainrate used for Md simulations / Lammps?

Shanique Murray

2

It appears you are just summing per-atom stress to get global stress.
Are you getting the same answer for press and v_press?

Getting negative pressure for tensile and positive for compressive is correct.
The magnitudes of pressure in MD are large b/c the systems are small
and the fluctuations are big.

Re: strain rate, you will always be using much bigger strain rates in
MD than you could do in expt. So there is no minimum, it just depends
on how long you can afford to run to get some change in your system at a tiny
strain rate.

Steve