Re: [ESPResSo] how to set up a charged plate

[Axel Arnold]

On Tuesday 23 June 2009 12:35, gil claudio wrote:
> Hi everyone,
>
> How does one set up a plate that is uniformly charged?
>
> For example, I have a box 100 x 100 x 100 with a constraint wall at the
> bottom of the box.  I would like to charge this wall uniformly with, say a
> total charge of +50.  How can this be done?

Hi! First let met note that charged walls _only_ work with either MMM2D or 
P3M/ELC with the noneutralization option of ELC; plain P3M will give 
completely unphysical results, since the charged walls in Espresso are not 
periodically replicated in the direction of their normal. Moreover, this 
normal has to be parallel to the nonperiodic direction of MMM2D and P3M/ELC, 
which is the third or z-axis. Therefore, you can only have a wall at the 
bottom or top of the simulation box.

Such a charged wall is defined by the command:

constraint plate height <h> sigma <sigma>

where height is the z-coordinate of the wall (in your case 0) and sigma the 
charge density (in your case 50/100x100).

Note that these are purely charged walls without volume interaction; 
typically, you also want to add a normal Lennard-Jones wall at the same place 
to avoid particles crossing the wall; since the particle-charged wall 
interaction energy is simply 2 pi q sigma |z|, there is nothing preventing 
particles from doing so.

Second note: if you introduce two identically charged walls at the top and 
bottom, you can as well not introduce them - again due to the interaction 
potential with a charged wall, they do not exert any forces on particles 
between them.

Axel

-- 
Dr. Axel Arnold 
Fraunhofer SCAI
Schloss Birlinghoven, 53754 Sankt Augustin, Germany
Tel: +49 2241 14 2575