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Re: [ESPResSo-devel] error in dawaanr interaction

From: Rudolf Weeber
Subject: Re: [ESPResSo-devel] error in dawaanr interaction
Date: Fri, 26 Sep 2014 11:24:25 +0200
User-agent: Mutt/1.5.21 (2010-09-15)

Hi Markus,
On Fri, Sep 26, 2014 at 10:30:11AM +0300, Markus Gusenbauer wrote:
> I am wondering if the equation for the dawaanr (dipole-dipole)
> interaction implemented in ESPResSo is correct
> (magnetic_non_p3m_methods.c).
> According to the following papers the force equation in the code is
> missing the term ยต0/(4 pi):
As far as I'm aware, all dipolar methods don't include this, neither in the 
force nor in the energy.
the reason is probably that the pre-factor depends on the unit system, you're 

You can set the pre-factor, at least in theory, using the Bjerum length
inter magnetic <l_b> dawaanr
The prefactor you get in front of forces and energies, according to the code, 
is (grep -i dprefac.*= src/core/*)
l_b * k_B T, if temperature >0 and just l_b otherwise.
So, if you set l_b=mu_0/(4*pi *k_B T), you should get the desired result.
I'm however not sure, if anyone has ever tested this with dawaanr.
With the dipolar p3m it was previously done.

Regards, Rudolf

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