Re: [ESPResSo-users] have_quaternion

[Rudolf Weeber]

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 
vs_quat
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