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Re: [ESPResSo-users] have_quaternion

From: Rudolf Weeber
Subject: Re: [ESPResSo-users] have_quaternion
Date: Mon, 4 Mar 2019 15:59:04 +0100
User-agent: Mutt/1.9.4 (2018-02-28)

Hi Martin,
On Mon, Mar 04, 2019 at 03:34:06PM +0100, Martin Kaiser wrote:
> >> I have two particles, one of them is a Langevin swimmer, the second is a 
> >> dipole. The dipole is a virtual site to the swimmer. My goal is to let the 
> >> dipole and the propulsion force always show into the same direction. This 
> >> means for me, that the director of both particles should show into the 
> >> same direction. The have_quaternion option should, to my understanding, 
> >> update the orientation of the virtual particle so it always has the same 
> >> orientation compared to the orientation of the real particle.
> >> 
> >> Unfortunately, as soon as I activate the have_quaternion feature, the 
> >> director of my virtual particle is set to [0,0,0]. I have no idea why that 
> >> happens and, in case it is intended, what it means.
The relative orientation of the virtual site and the base particle is 
crontrolled by the virtual sites 
property. If you want co-alignment, you have to set it to (1,0,0,0), which is 
the identity for quaternions.
The vs_quat-property defaults to (0,0,0,0), which results in a 0-director and 
q0-quaternion on the virtual sites if have_quaternions is true.

I had a look at the testcase, and think that the current implementation is 
working, once vs_quat on the virtual site is set.

Regards, Rudolf

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