> What would the user want the information for? (Sorry if this has been
> explained before, but as I said I didn't follow the early part of this
> thread, and although I looked at the archive I may have missed
> something.)
>
> I take it that the user might need the information to reproduce the
> same view again,
Correct.
> or to drive some other program to produce the same
> view.
Yes.
> In that case RxRyRz information would be useful only if JMol or
> some other program could understand it as input, and that seems
> unlikely.
Not true.
RasMol, Chime, Jmol all support:
rotate [x|y|z] {degrees}
I suspect that any other scriptable system would support the same.
> Jmol could presumably interpret 3 successive rotations, but in
> that case it would cope just as well with RzRxRz or RzRyRz.
Agreed.
> The
> transformation to Euler angles is singular if the rotation is around
> the z axis, so there are numerical problems if the rotation axis is
> close to the z axis, though in practice that can be handled as a
> special case.
I understand this in general, but not the specifics.
> The advantage of the angle-axis representation is that
> it can be obtained straightforwardly from the rotation matrix via the
> quaternion representation.
Internally, things are done with the matrices. And I use AxisAngles when
appropriate.
> If Euler angles are to be used, I would advocate RzRyRz, which is used
> in quantum mechanics and hence in most applications concerning
> molecules, rather than RzRxRz, which is mainly used in classical
> mechanics. But the relationship between them is very simple so it's
> not a big issue.
>From my perspective, this is *only* used for informational purposes ...
and in case the user wants to use something other than an AxisAngle to
return to a former orientation.
But, it makes sense to me that we should output in a format that is
generally used, is documented, and will be familiar to at least some
practitioners.
So, if the quantum mechanics folks tend to use RzRyRz, then that makes
sense to me.
Miguel
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www.jmol.org
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