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Re: [ESPResSo-users] problem with magnetic particle system

 From: Rudolf Weeber Subject: Re: [ESPResSo-users] problem with magnetic particle system Date: Wed, 15 Aug 2012 17:05:45 +0200 User-agent: Mutt/1.5.21 (2010-09-15)

```Hi Judith,
On Wed, Aug 15, 2012 at 03:29:43PM +0200, address@hidden wrote:
> I`m trying to simulate a simple magnetic particle system in a solution.
> Although I read the manual I am having problems making the simulation
> work.
>
> At the moment I am trying to use the "inter magnetic lB p3m ( tune |
> tunev2 ) accuracy accuracy" - command to simulate the dipolar interaction.
> What does accuracy mean in this context? Is it related to the energy?
I think, it is the ABSOLUTE force error estimate.
Note that the estimate is based on the assumption of a homogeneous particle
distribution. If your system is highly inhomogeneous, the actual accuracy may
be worse than the one you specified.

When you use kT=1, Lennard-Jones sigma=1, and lambda=m^2/(kT sigma^3)<5 or so,
you can try 10^-4 for the accuracy.
(m=Dipole moment).

>
> During the tests we got the following error message:
> "dipoar fft - Complex value is not zero"
> Under which circumstances does this error occur?
I only ever got this when there were extremely few particles in the system.
Could you provide your script, so that we can try to figure out, why this is
caused?

> It would be great if someone could point me to a simple example script for
> magnetic nanoparticle interaction.
Apart from the usual setup like thermostat, skin, box size and time step, one
needs to assign the particles a dipole moment in vector form like
part 0 pos 1 1 1 dip 1 2 3
When you draw the dipole moments randomly, make sure that they are equally
distributed on the uint sphere. E.g., you can draw phi in [0..2pi] and
cos(theta) in [-1..1].

Then, you need to setup p3m like
inter magnetic \$bjerum p3m tunev2 accuracy \$accuracy

If you need a full script, let me know.

Hope that helped!
Regards, Rudolf

```