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Re: quaternions

From: Felix Höfling
Subject: Re: quaternions
Date: Wed, 18 Dec 2019 15:41:46 +0100
User-agent: Opera Mail/12.16 (Linux)

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?

No thought yet. It is true that for axisymmetric bodies, quaternions are not necessary. For quaternions, I see no issue with the name "quaternion", it is unambiguous and actually used internally (even though "q" or "quat" would be the variable name, the data is named quaternion in the code and documentation such
as "the quaternion" as one would say "the position").

What I find a bit weird about the name "quaternion" is that it refers to the data type, just as "vector" or "complex number", and not to the property it describes. Admittedly, I don't know about any other use of quaternions than for representing rotations, at least in MD.

By the way, "rotation" would be a nice name too. How is it named in the LAMMPS or Espresso interfaces (not necessarily the code)?

What you refer to as axis is the unit vector pointing along the "long" axis of the ellipsoid, right? If so, I can think of "axis", "u", "unit_vector", or even "orientation" actually. All current names in the particles group (even the new quaternion) are unambiguous. Apart from "id", we even have full english names
for all elements :-)

"axis" sounds good, or "orientation"

Practical question: is it required (in H5MD) that the quaternions/axis vectors are normalised? Should it be stated?


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