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Re: [ESPResSo-devel] error in dawaanr interaction

From: Markus Gusenbauer
Subject: Re: [ESPResSo-devel] error in dawaanr interaction
Date: Fri, 26 Sep 2014 15:59:49 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.4.0

Hi Rudolf,

thanks for your fast answer. When i searched for dprefac*, I couldn't find it in the computation of dawaanr. Just with ROTATION, which I don't use. Now I am curious again, where does l_b act in the code in case of dawaanr? Also when I search for bjerrum, it is also not acting for dawaanr interactions.


On 26.09.2014 12:24, Rudolf Weeber wrote:
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

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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