# pressure of a region, determining & barostatting

Dear all

I have a bi-part question,

1. how can I determine the pressure of region? (not a group of atoms)

2.I want to set the pressure of a specific region and keep it constant.
the Nose-Hoover & Berendsen barostats fix the pressure of a specific group (not a region).

however I want to keep constant the pressure of a reservoir included fluid transfer in and out.

is there any way to fix its pressure?

Note that the boundaries of the reservoir are flexible and it is not important whether its volume changes or not.

best regards
Fahim

Dear all

I have a bi-part question,
1. how can I determine the pressure of region? (not a group of atoms)

how is pressure of a region defined?

2.I want to set the pressure of a specific region and keep it constant.
the Nose-Hoover & Berendsen barostats fix the pressure of a specific group
(not a region).

however I want to keep constant the pressure of a reservoir included fluid
transfer in and out.

is there any way to fix its pressure?

this sounds quite a bit like an ill conceived simulation,
like trying to mimic a macroscopic experiment where
it would not at all be needed on the microscopic scale,
since there are alternate ways to achieve the same effect.

Note that the boundaries of the reservoir are flexible and it is not
important whether its volume changes or not.

what are those boundaries made of then?
how would you represent them in the simulation?

axel.

Steve

Dear all

I have a bi-part question,
1. how can I determine the pressure of region? (not a group of atoms)

What is the pressure of a region, other than the pressure of the atoms
within it? You can compute per-atom stress via compute stress/atom
(which is related to pressure), and sum it over atoms within a region
via compute reduce/region.

2.I want to set the pressure of a specific region and keep it constant.
the Nose-Hoover & Berendsen barostats fix the pressure of a specific group
(not a region).

however I want to keep constant the pressure of a reservoir included fluid
transfer in and out.

is there any way to fix its pressure?

If atoms are free to move in/out of a region, how would you
imagine controlling their pressure. Pressure is typically controlled
(e.g. in an NPT ensemble) which adjusts the volume of the simulation
box, so that the atoms in it feel more/less contrained. How would

Hi

I followed the Steve’s suggestion and computed the pressure via
stress/atom and reduce/region with also some minor post processes.
so thanks.

however, the barostatting is still ambiguous.
let me describe the process more explicitly:

sorry about the volume actually I mistook!
the volume and dimensions of the reservoir are constant (exactly the definition of the region-command)
and I am going to keep constant the pressure of fluid inside the reservoir while the number of internal atoms
is varying during the time.
keeping in mind the general form of the equation of state: P=P(V,N,T)
V (volume) is constant originally
P (pressure) is constant on desire
and N & T are variable, thus P=P(N,T)
so I’m looking for a command scaling the atoms velocities artificially so that
the amounts of N,T varying dependently until the pressure remains constant.

best,
Fahim

Sounds like you just want a thermostat and to monitor
the pressure. There is no way to set a thermostat so
that the system converges on a desired pressure.

Steve