compatibility of oblique cell with electrostatic code

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1

Hi,

I am trying to simulate brwownian dynamics of dipole suspensions under shear. I use fix deform to shear the simulation box. Is this a legitimate thing to do? In other words, are the Lees-Edward BCs i.e. oblique cell compatible with electrostatic code? I appreciate your response.

Best,
Hamed

2

Hi,

I am trying to simulate brwownian dynamics of dipole suspensions under
shear. I use fix deform to shear the simulation box. Is this a legitimate
thing to do? In other words, are the Lees-Edward BCs i.e. oblique cell
compatible with electrostatic code? I appreciate your response.

as has been pointed out repeatedly on this list, LAMMPS does not do
Lees-Edwards boundary conditions.

axel.

3

However, what fix deform does is essentially equilvalent
to LE boundary conditions, and electrostatics work fine
with fix deform.

Steve

4

Hi,

My simulations show that most likely the electrostatic code does not work fine with fix deform i.e. it seems there is a localized shear strain at the box boundaries. This might be a bug.

I am attaching a movie with 9 periodic cells and my scripts. Please look how chains break at the boundaries due to a localized shear strain which is not present inside the simulation box. Here are my ingredients:

  • Polarized particles with 20% volume fraction and zero charge (i.e. dipole interactions)
  • A repulsive WCA core
  • External field is on
  • Suspension is under a constant shear rate (i.e. fix deform)
  • Brownian forces at room temperature

Best,
Hamed Abdi

model.txt (1.16 KB)

data.txt (8.91 KB)

oblique cell.zip (6.07 MB)

5

Hi,

My simulations show that most likely the electrostatic code does not work
fine with fix deform i.e. it seems there is a localized shear strain at the
box boundaries. This might be a bug.

what makes you think this is due to electrostatics?

from superficially looking at your input, it seems more likely to me
that you need to re-read some of the "IMPORTANT NOTE:" sections in the
fix deform documentation.

axel.

6

The Kspace styles should all work fine with a deforming box,
for orthogonal or triclinic shaped boxes.

Steve

7

I remap velocities and use a Langevin thermostat which according to the documentation should be fine. But the problem is still there. There is a discontinuity in shear strain at the boundaries. I agree it is not necessarily from electrostatics but something looks not correct.

Hamed

8

I remap velocities and use a Langevin thermostat which according to the
documentation should be fine.

that is not how i read the documentation. it is very straightforward
and explicit. and it just makes sense, if you think about it: since
fix langevin is a dissipative thermostat , you *have* to include a
bias consistent with your deformation, or it will mess up your
dynamics since it will try to dissipate your velocity gradient (and
the corresponding temperature gradient).

axel.