Constant area simulations with lammps

I’ve found a post concerning “Constant area simulations with lammps” (http://lammps.sandia.gov/threads/msg30742.html).

Can’t we make fluctuating area by having xy coupled in npt ensemble?

Like people used to do for a lipid membrane simulations (ex. Biophisical Journal 92, 147-161, 2007)? In my lammps file I have eg.:

fix 2 copol npt temp {T} {T} 100.0 x {p} {p} 500.0 y {p} {p} 500.0 z {p} {p} 500.0 couple xy

and a cross area is fluctuating around a constant value.

My question is rather how to implemented in LAMMPS a simulations with constant x, constant y and fluctuating z for a given pressure. In the post I have mentioned, there is that: fix 1 all npt temp 300 300 100 z 500 500 1000.0 will have z fluctuating and x constant and y constant. But I do not know how these x and y values are established? The only way that comes to me now, is that I will need to have nvt ensemble with established x, y, and z and then switch to this npt (fix 1 all npt temp 300 300 100 z 500 500 1000.0), but it sounds little bit awkward…

Could you tell me how to do the simulations where you have constant x, constant y and fluctuating z so the pressure is constant?

Regards,

Goska Kowalik

Can’t we make fluctuating area by having xy coupled in npt ensemble?

See the couple xy option of fix npt.

If you want constant x and y (as you also indicate below),
then only barostat on z.

Steve