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mrkllntschpp / lammps-tutorials

LAMMPS tutorials for Beginners
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confusion with strain rate units #2

Closed sphCow closed 4 years ago

sphCow commented 4 years ago

Before I begin, thanks a lot for making these tutorials available. I've a doubt here. In tutorial 3, after the initial equilibration, the simulation box is deformed

in the x-direction at a strain rate of 10^10 1/s

That is implemented in the LAMMPS script in the following lines

units       metal
...
timestep 0.001  #in picosecond 
... 
variable srate equal 1.0e10
variable srate1 equal "v_srate / 1.0e12"
fix 2 all deform 1 x erate ${srate1} units box remap x

How do you calculate the factor of 1.0e12 ? Isn't the timestep dt involved here in some way?

claireb5112 commented 4 years ago

Hello! I think it is because the pico prefix value is 1e-12. So dividing the srate by 1e12 (or multiplying by 1e-12) you are converting srate in sec to pico sec. So your total time for the calculation is 10^10 seconds (1e10) and you are having the time step be a pico second. ? Hopefully the writer of this code can verify, but those are my thoughts.

mrkllntschpp commented 4 years ago

Thanks! I went ahead and added this to the FAQ section. Claireb5112, you were almost right on all accounts. It is a unit conversion. The total time isn't 10^10, that is just the strain rate; and the timestep isn't picosecond, but rather 0.001 picosecond (femtosecond, 1e-15 s). I added a little more explanation in the FAQ.