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

[Tom Daff]

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.

 * 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.

 * 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.

 * 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