Re: [ESPResSo-users] Few quick questions. Electrostatics, LB.

[Axel Arnold]

Hi!

On 03.02.2015, at 07:45, Harmanjit Singh <address@hidden> wrote:

What is best way to make two point charges kept ~10- 50 molecular distance unit apart to interact electrostatically? Which algorithm should be applied?

The algorithm you need depends on the boundary conditions that apply to your system. 3d periodic boundary conditions require P3M, 2d+h-periodic bc require MMM2D or ELC + P3M. Nonperiodic bc require to use the Debye-Huckel potential as bare Coulomb interaction.

I am simulating a system where I have lined an array of negatively charged particles on the boundaries of the simulation box ( I realized I could not use charged plates because constraint plate works only in z direction, or is there a better way to do this?). There are a few charged molecules in the box, but they do not move at all during simulation as if they are not affected by any electrostatic field at all. I have tried p3m, mmm2d and played around with the arguments, without luck.

That is correct, if periodic boundary conditions apply perpendicular to your charge array. As a charge array exerts a constant force up to sign, you don’t get any force with two like-charged parallel plates, which is what periodic boundaries make out of your setup. You might first want to rethink the physics of your system...

Btw, that is also the reason why the plate constraint only supports being perpendicular to the z-axis. If you use the plate, you necessarily need periodic boundary conditions parallel to the plate (otherwise the used formula is wrong), but you cannot have periodic boundary conditions in perpendicular direction. That leaves only  MMM2D and ELC as algorithms, which both use the z-coordinate as  the nonperiodic coordinate.

Best,

Axel

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JP Dr. Axel Arnold
ICP, Universität Stuttgart
Allmandring 3
70569 Stuttgart, Germany
Email: address@hidden
Phone: +49 711 685 67609