[ase-users] Update to IUPAC 2016 elemental weights - discussion

[Jens Jørgen Mortensen]

On 12/08/2016 09:37 AM, Tristan Maxson via ase-users wrote:
> I have been watching this issue for a while,  even if the values are 
> updated I believe we should keep the old values around for backwards 
> compatibility.  A environment variable could also be available for 
> picking a default set if need be (defaulting to the most recent).  
> This would help with keeping consistent values during a project 
> without having to modify all of the scripts if a new version of GPAW 
> is being used (either out of need or wanting to simply use up to date 
> software).  This could also be useful for changing default sets for 
> people that have different needs (average isotope vs most common).

Thanks for your feedback, all of you.  I think everyone agree that we 
should have the new numbers in ASE.  Let's continue the discussion here:

     https://gitlab.com/ase/ase/merge_requests/360

Jens Jørgen

>
> ASE_ISOTOPES=IUPAC-Nov2016
>
> Thank you,
> Tristan Maxson
>
> On Wed, Dec 7, 2016 at 8:50 PM, Rasmus Karlsson via ase-users 
> <ase-users at listserv.fysik.dtu.dk 
> <mailto:ase-users at listserv.fysik.dtu.dk>> wrote:
>
>
>     Tom Daff via ase-users writes:
>
>     > Dear Users,
>     >
>     > As suggested by Jens, feedback is greatly welcomed on a merge
>     request
>     > that I submitted recently:
>     >
>     > https://gitlab.com/ase/ase/merge_requests/360
>     <https://gitlab.com/ase/ase/merge_requests/360>
>     >
>     > IUPAC regularly revises the names and atomic weights of the
>     elements.
>     > Their most recent publication on the weights can be found in
>     Pure and
>     > Applied chemistry (https://doi.org/10.1515/pac-2015-0305
>     <https://doi.org/10.1515/pac-2015-0305>). The masses
>     > in ase have been the same since the beginning, but I have proposed
>     > updating them to be in line with the most up-to-date values.
>     There are a
>     > few issues which should probably be discussed first:
>     >
>     >  * Changing the weights will change the results of some
>     calculations.
>     > Most values are the same to 4 or 5 significant figures, however
>     changing
>     > the weights will cause slight changes in things like MD, centre
>     of mass,
>     > phonons. Should anything be done about this? Some of the calculator
>     > tests may be affected too.
>     >
>
>     The most recent values should be used. If tests break, they should
>     be fixed.
>
>     >  * Some atomic weights are expressed as intervals. The masses of
>     > isotopes can be measured more precisely than their relative
>     abundances
>     > so a range of expected values is given. IUPAC also has
>     "conventional"
>     > values which are to be used where an interval can't be used. In some
>     > cases these end up with fewer digits of precision than ase already
>     > included (e.g H was previously 1.00794 but is actually defined
>     as the
>     > interval [1.00784, 1.00811], with a conventional value of 1.008). I
>     > think that the conventional values are the correct ones to use
>     for these
>     > cases.
>
>     Agree.
>
>     >
>     >  * How should these be updated in future? The weight of
>     Ytterbium will
>     > be changed next year, and the weights could be revised every few
>     years.
>     >
>
>     They should be updated when IUPAC does so, and as was noted by Eric
>     Hermes, changes should be made clear in the changenotes.
>
>     >  * Is NaN still the correct value to use for elements without a
>     > statistically defined weight? Since NaN's can propagate
>     silently, you
>     > can end up with nonsense results which are hard to track down. None
>     > would be more likely to raise an error, or errors could be
>     raised when
>     > accessing missing weights, or the mass of an isotope could be
>     added as a
>     > place-holder instead.
>     >
>     > Thanks,
>     >
>     > Tom
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>
>     --
>     Rasmus Karlsson, PhD
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