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


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!



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:


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


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.