applying NγT ensemble in lammps

Hello everyone

I’m modeling a bi-layer system with vacuum in both sides of it and I need to use NγT (constant surface tension) ensemble, since LAMMPS doesn’t have this ensemble I have to use NPT to fix pressure tensor in x and y directions in order to reach the desired surface tension, but according to surface tension relation : gamma=1/2(pzz-1/2(pxx+pyy)) i need to evaluate pzz within simulation, to evaluate pxx and pyy , so I should change pstart and pfinal constantly! And NPT doesn’t have any option for this. does LAMMPS have another option for this?( I can’t impose any value to my pzz because I need the vacuum upside it)

thanks for the answer


Hi Ali,

You do not need to change Pxx and Pyy constantly. Pressure fluctuates a lot, and that would likely just cause an unneeded headache (and maybe system issues).

If you just want to have a constant surface tension, then use NPT with the aniso pressure option. You can set the Pzz to a constant pressure (again, this will fluctuate a lot but should be what you set it to on average) and set your Pxx and Pyy pressures based on Pzz to get your desired surface tension. You can use the surface tension equation to determine appropriate Pxx and Pyy values.

Kind Regards,