Re: [h5md-user] Finalization of units module

[Konrad Hinsen]

Felix Höfling writes:

 > Thinking about the "system" attribute again during the vacation I'm a bit
 > unsure whether it is really needed. In particular given the fact that it
 > can (and must) take a single value only.
 > 
 > Extending the set of units could be achieved by simply increasing the
 > version number of the units module.

The underlying question is: do we want a single unit system, able to grow
as new needs are identified, or do we envisage distinct unit systems
that are completely independent?

I'd go for the second approach, and keep the "system" attribute, even
if initially only "SI" is defined. The major alternative unit systems
for which I see a need are domain-specific unit systems defined for
theoretical models. A simple example is "reduced LJ units" but there
are more elaborate ones. I see them arriving in coarse-grained models
for macromolecules. These unit systems have no reference at all to the
real world (and thus to SI), and may use arbitrary symbols.

Finally, a minor point but I think it's worth considering. The SI
system defines a fixed set of units and prefixes, plus some tolerated
units that don't fit the system but are convenient, such as liters for
volume.  A major goal of SI is simplification through standardization
and thus the elimination of older units.  A unit system designed to
include lots of other units as long as no name conflict arises should
not be called "SI".

Konrad.
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Konrad Hinsen
Centre de Biophysique Moléculaire, CNRS Orléans
Synchrotron Soleil - Division Expériences
Saint Aubin - BP 48
91192 Gif sur Yvette Cedex, France
Tel. +33-1 69 35 97 15
E-Mail: research AT khinsen DOT fastmail DOT net
http://dirac.cnrs-orleans.fr/~hinsen/
ORCID: http://orcid.org/0000-0003-0330-9428
Twitter: @khinsen
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