I'm having really hard time finding a way how to implement NPAT ensemble (constant normal pressure and cross sectional area) for ultimately calculating the surface tension obtained from the mechanical definition. I read the mail list and the manual, so tried many ways such as the command lines below but the cross sectional area will not stay fixed or the box length in z direction keeps increasing. I was wondering if anyone could help me with this issue and let me know if there is a specific way of applying NPAT ensemble in LAMMPS. I really appreciate the time and consideration.
fix fixnpt all npt temp 298.15 298.15 2000 x 0 0 2000 y 0 0 2000 z 98.6923 98.6923 2000 couple xy drag 20.0
fix fixnpt all npt temp 298.15 298.15 2000 aniso 98.6923 98.6923 2000 couple xy drag 20.0
fix fixnpt all npt temp 298.15 298.15 2000 z 98.6923 98.6923 2000 drag 20.0