Re: [ESPResSo] Dipoles and the Gay-Berne Potential

[Axel Arnold]

Am Donnerstag, 15. Oktober 2009 18:37 schrieb Michael Winokur:
> Hi,
>
> I've now started looking at the Gay-Berne subroutine gb.h in order to
> understand how these particles are oriented.  From what I can deduce gb.h
> uses the quaternion representation of the orientation for calculating pair
> interactions.   I note that the "quat" parameter is not well documented is
> the manual.  In addition there are functions (in rotation.c and rotation.h)
> which move between the dipole orientation, phi & theta and the quanternion.
> (By the way, address@hidden is the e-mail address given in
> gb.h).  Is it not clear to me which comes "first", the dipole (dip) or the
> quaterion (quat).  It would seem that only one of these is needed by gb.h.

Hi!

Yes, as I wrote, you only need the quaternions; the dipole moment is always 
made sure to be oriented along the main axes of the quaternion 
representation. The dipole moment is not used for GB, but is necessary for 
magnetostatics.

The problem is, that writing the electrostatics code is much easier not on 
quaternions, but on the director; however, the integration requires the 
quaternions.  Therefore, both representations exist, but the quaternion one 
is the authoritative; at the beginning of the force calculation, the 
quaternions are converted to dipole directors. If you set the dipole moment 
of a particle explicitely in Tcl, then of course this dipole moment is 
converted and into a quaternion.

Regarding the mail address: Dmytro Antypov, the creator of the rotational 
code, has long left the institute, so at present, there is no one that is 
really familiar with the rotational code.

Axel

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