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

[Jakob Schiøtz]

> On 07 Dec 2016, at 13:12, Tom Daff via ase-users <ase-users at listserv.fysik.dtu.dk> wrote:
> 
> 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.

My personal opinion is that ASE should be updated to reflect the modern values, and the affected tests should be updated as well.  If the test was passing before, and we know why the values change a bit, then this should be OK.

> * 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.

Agreed.

> * 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.

I think we should occasionally update the weights.  Most of them probably will not change.

> * 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.

NaN is evil :-)

For elements that does not exist in nature, using the value for the most long-lived isotope must be the most sensible, just like most periodic tables usually do. 

Best regards

Jakob



> 
> Thanks,
> 
> Tom
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--
Jakob Schiøtz, professor, Ph.D.
Department of Physics
Technical University of Denmark
DK-2800 Kongens Lyngby, Denmark
http://www.fysik.dtu.dk/~schiotz/