# Can we control pressure in the Lee-Edwards BC (fix deform with a triclinic box in Lammps)

Dear Lammps users,

I am working on a project, where I am trying to generate a simple linear flow with Lees-Edwards Boundary Conditions in NPT esemble. For NVT system, the Lees-Edwards BC can be implemented by fix deform with a triclinic box using fix nvt/sllod in Lammps. However, I also would like to maintain the system’s pressure at a constant value. Is it possible to give me some advice on how to solve this issue?

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Best wishes,

Jesica Law

Dear Lammps users,

I am working on a project, where I am trying to generate a simple linear flow with Lees-Edwards Boundary Conditions in NPT esemble. For NVT system, the Lees-Edwards BC can be implemented by fix deform with a triclinic box using fix nvt/sllod in Lammps. However, I also would like to maintain the system’s pressure at a constant value. Is it possible to give me some advice on how to solve this issue?

pressure will fluctuate for as small a system size as it is typical for MD simulation.
you can select the average pressure through choosing your simulation volume, i.e. if you want to simulate a flow at higher pressure, equilibrate your system for that pressure or select the density accordingly and then you’ll get (on average) a different pressure. a “constant” pressure would be unphysical.

axel.

The fix npt command gives you options to barostat different dimensions
of the simulation box, including for triclinic boxes. If you are shearing in
x, relative to y, (what LAMMPS calls an xy shear), then you
can certainly barostat z. You can probably also barostat y, though
adjusting the y dim on-the-flly will alter the xy shear metric. So it might compete
with the continuous shear you are applying. Likewise if you
barostat x. So I’m not sure what box dimensions you want to

control for a continuously shearing system? For thermostats, people
will sometimes only thermostat the non-shear directions to avoid this issue,
though nvt/sllod is a more formal way to do it.

Steve