Swelling of PMMA in supercritical co2 – LAMMPS Beginners

Hi all,

I want to study the swelling behavior of PMMA in supercritical co2(a bundle of PMMA is placed in the center of co2 box). I want to run an NPT simulation at 250 atm. since co2 molecules should be treated as rigid molecule, I have to define separate group for both co2 and PMMA. how can I maintain the pressure of the system.
instead of co2 if it was a flexible molecule , I could have defined ” fix NPT for group all”.

please share your valuable suggestions.

Thank you


Hello,

The documentation offers 3 solutions:

If you wish to perform NPT or NPH dynamics (barostatting), you cannot use both fix npt and the NPT or NPH rigid styles. This is because there can only be one fix which monitors the global pressure and changes the simulation box dimensions. So you have 3 choices:

  1. Use one of the 4 NPT or NPH styles for the rigid bodies. Use the dilate all option so that it will dilate the positions of the non-rigid particles as well. Use fix nvt (or any other thermostat) for the non-rigid particles.
  2. Use fix npt for the group of non-rigid particles. Use the dilate all option so that it will dilate the center-of-mass positions of the rigid bodies as well. Use one of the 4 NVE or 2 NVT rigid styles for the rigid bodies.
  3. Use fix press/berendsen to compute the pressure and change the box dimensions. Use one of the 4 NVE or 2 NVT rigid styles for the rigid bodies. Use fix nvt (or any other thermostat) for the non-rigid particles.

In all case, the rigid bodies and non-rigid particles both contribute to the global pressure and the box is scaled the same by any of the barostatting fixes.

Simon



1 Like

Do you really have to use fix npt?

Supercritical systems are notoriously difficult because of the large local fluctuations that happen in the supercritical state. They are particularly large around the critical point and it is thus generally recommended to perform MD simulations sufficiently away from the critical point. To further minimize problems, I would just run the system at fixed volume with a suitable density and temperature. That may not give you the exact desired pressure, but you will be in the ballpark and you bypass a lot of problems due to the fixed volume. If the system is sufficiently large, there should be no significant difference in the thermodynamical results due to the different statistical mechanical ensembles, at least none that are larger than the statistical errors and systematic errors from using a force field. …and (again) with the simulation in the supercritical CO {}_2 region those statistical errors would be very large for a small system, anyway.

  1. Use one of the 4 NPT or NPH styles for the rigid bodies. Use the dilate all option so that it will dilate the positions of the non-rigid particles as well. Use fix nvt (or any other thermostat) for the non-rigid particles.
  2. Use fix npt for the group of non-rigid particles. Use the dilate all option so that it will dilate the center-of-mass positions of the rigid bodies as well. Use one of the 4 NVE or 2 NVT rigid styles for the rigid bodies.
  3. Use fix press/berendsen to compute the pressure and change the box dimensions. Use one of the 4 NVE or 2 NVT rigid styles for the rigid bodies. Use fix nvt (or any other thermostat) for the non-rigid particles.[quote=“simongravelle, post:2, topic:46857, full:true”]
    Hello,

Thanks for the suggestions. I still have a small doubt.
if we consider the first point listed above, npt is defined on rigid group and nvt for flexible group. one can change box dimension, and fix nvt will try to keep the volume constant. how can both occur simultaneously.

i was also preferring this method. but I didn’t succeed in finding the necessary density that will give me sufficient high pressure. then I shifted my thoughts to NPT simulations.

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