# Pressure increasing after some steps in NPT

I am using periodic boundary condition in two directions and fixed boundary condition in x and y direction. Then I am heating using NPT and applying barostat in the two periodic directions. Also I use fix wall/reflect so that atoms do not leave the simulation box in fixed direction.

I see that at low temperatures the pressure oscillates around the required pressure, but at high temperatures the pressure increases too much.

fix 3 all npt temp 0.1 3500 1.0 x 1.0 1.0 1.0 y 1.0 1.0 1.0 couple none

What can be the possible reason for it?

I am using periodic boundary condition in two directions and fixed boundary
condition in x and y direction. Then I am heating using NPT and applying
barostat in the two periodic directions. Also I use fix wall/reflect so that
atoms do not leave the simulation box in fixed direction.

I see that at low temperatures the pressure oscillates around the required
pressure, but at high temperatures the pressure increases too much.

fix 3 all npt temp 0.1 3500 1.0 x 1.0 1.0 1.0 y 1.0 1.0 1.0 couple
none

What can be the possible reason for it?

how do you determine that it is "too much"?

axel.

I plot a graph of pressure with timesteps . At iniital low temperatures pressure is the required value but as temperature increases the pressure too increases and graph slope changes abruptly.

Also there is typo in previous mail. I use periodic boundary condition in x and y directions and fixed in z direction

Well it looks like you’re ramping the temperature to 3500 K, its quite likely your barostat will have an “oscillating” system pressure much larger than the 1.0 pressure unit you specified. Even with common temperatures like 300K I’ve seen oscillations over an order of magnitude higher when I specify 1.0. When you take an ensemble average ,or at least a time average, over many states of the phase space the value should converge to 1.0 for the system pressure.

I plot a graph of pressure with timesteps . At iniital low temperatures
pressure is the required value but as temperature increases the pressure too
increases and graph slope changes abruptly.

that doesn't mean anything unless your rate of ramping up the
temperature is so slow that your system will remain in a near
equilibrium state.
furthermore, it is generally considered good practice to assume longer
relaxation time for the barostat (10x to 100x).
also, you didn't say over what size window you are averaging
temperature and pressure over.

axel.