Do you mean that how to pressure only in one direction, like Z ?

Maybe you can use command like this

fix 1 all npt temp 300.0 300.0 100.0 z 1.0 1.0 100.0

Thanks Gao

But how to fix the area normal to z direction at the same time?

Bests

If you use the barostat only in one direction (as Gao demonstrated), the box dimensions in other directions essentially remains constant. So, if you only fix P_z, LAMMPS only changes the height of the simulation box, with its length and width fixed.

One other option that comes to mind for measuring interfacial tension is to consider bubbles of one fluid inside another. You would simulate the whole box with constant pressure, and it should results in constant normal pressure on the bubble surface.

Thanks Mohsen

Have you got experience in calculating IFT through lammps?

I heard of some difficulty in dealing with pressure fluctuation in this process. and why we should use NPnAT for that. I can't catch it!