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

[Rasmus Karlsson]

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
>
> 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).  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