Re: quaternions

[Felix Höfling]

Hi Pierre,

Very good idea. It is also pleasing to see that H5MD is alive - from daily
experience it works great :-) And from recent discussions on a conference
it seems that there is increased interest from more groups as well.

Specific comments on the quaternions: The storage order (scalar part,
vector part) follows also the notation I'm familiar with from the
mathematical literature. And of course, the big packages leave no other
choice.

Where should it go? I suppose it would be listed as a property in the
particles/ tree, next to "position"?

My own HALMD software is close to support quaternions as well. Here a
question came up, which we may address now: What is the relation of the
quaternion to the axis vector for axisymmetric particles (e.g.,
ellipsoids)? A natural name for both properties would be "orientation" -
so how should we distinguish them in H5MD? Of course, we could call the
quaternion representation "orientation" and the axis representation "axis"
or "direction" or so. Any thoughts on this?

Best wishes,

Felix