[lammps-users] A problem about the pressure control in NPT ensemble

Hi

I would like to equilibrate a system of amorphous SiC bulk at 300K for hundreds of ps. I used the command " fix 1 all npt 300.0 300.0 0.01 xyz 1.0 1.0 0.2 drag 0.7". Although the variety of temperature was in the range of 10K, the pressure fluctuated with thousands of bars. I tried to replace the p-style “xyz” with “aniso”, it yet didn’t work. Is it the problem about the Pdamp? How to choose a proper Pdamp?

Another question is about the uniaxial tension. I think it’s a non-equilibrium dynamic simulation, isn’t it? I want to use the command" fix deform" without controlling the pressure of z axis, provided that the extension is along with z axis. Should I add the term" rmap v" at the end? And another temperature compute " compute temp/deform" needs to be applied? Finally, the component of pressure tensor Pzz is the true stress of z axis?

Thanks a lot!

kun

Kun,

Large fluctuations in pressure are quite common in molecular dynamics simulations. But your damp values seem awfully small. Assuming you are using real units, you might want something more like:

fix 1 all npt 300.0 300.0 100.0 xyz 1.0 1.0 1000.0 drag 0.7

Please see:

http://lammps.sandia.gov/doc/fix_npt.html

And also make sure that you’re using the current version of LAMMPS:

http://lammps.sandia.gov/download.html

As far as fix uniaxial goes, yes, it was for non-equilibrium simulations. Note that the uniaxial fix no longer exists in the current version of LAMMPS. (See: http://lammps.sandia.gov/bug.html , 22 Jun 2007: "IMPORTANT: […] The fix volume/rescale and fix uniaxial commands were deleted since their functionality is now present in the fix deform command. " For your remaining questions, please carefully read the fix deform documentation: http://lammps.sandia.gov/doc/fix_deform.html and then let us know if you have further questions.

Paul