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

 From: jmeyer Subject: Re: [ESPResSo-users] problem with magnetic particle system Date: Thu, 16 Aug 2012 16:14:56 +0200 User-agent: SquirrelMail/1.4.20

```Hi Rudolf,

thanks for the quick response.

> 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).

Thanks, I tried to adapt the accuracy and the Bjerrum length to the order
of my dimensions.

>> 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.

When I now try to run the script I get a segmentation fault in the main
integration
loop. I am using Espresso on mac. I have installed the current version
fftw3.3.2 from
macports and gcc version 4.7.

Regards,
Judith

```

dipolsystem.tcl
Description: Tcl script