NPT - Achieve specific stress state

Hello all,

I am trying to achieve a specific stress state using NPT run. The simulation (porous metal) box is subjected to minimization, NVE equilibration and NPT equilibration. Triclinic box conditions are used.
The simulation box has to achieve the following stress state.
fix 1 all npt temp 300 300 0.3 x 0 212.89 3 y 0 3383.8 3 z 0 590.47 3 xy 0 0 3 yz 0 0 3 xz 0 0 3
timestep 0.003
run 1000000

However, the average values are far away from the fix npt.
Kindly assist.

There is no useful information here about your system (or your LAMMPS version and platform) and more to make any kind of assessment of what is happening and thus come up with some suggestion. Also, there is no meaningful quantification of your results. “Far away” may not be as far as you think it is.

But with a fix npt command such as the one you are using, you are creating a ramp for the barostat, no? The target pressure it tries to restore at each time step is not really the Pfinal, but rather an intermediate value, in-between Pinitial and Pfinal, corresponding to that given time step. So if you calculate the average Pij values over these 1m timesteps, you are indeed not going to obtain the respective Pfinal values you want. In fact these average values wouldnt even have any meaning within statistical mechanics as “the macroscopic pressure of your system”, given that your system is not equilbrated.

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