What does it mean for _diffrn_ambient_temperature and _cell_measurement_temperature to be different?

[James H]

Thank you all for your insights and information.

I think the best way forward is simply to clarify in the definitions
that the cell_measurement_temp/pressure items would only differ
(slightly) from _diffrn_ambient_temp/pressure where the cell was
determined separately from the diffraction pattern. If these items are
absent their default values can be taken from the corresponding
_diffrn_ambient items. Making the reverse deduction can be a matter of
policy for CIF curators.

If there are no objections, I will make the updates to the core
dictionary. You can follow along and comment here:
https://github.com/COMCIFS/cif_core/issues/294

On Tue, 18 Jan 2022 at 20:19, Spek, A.L. (Ton) <A.L.Spek at uu.nl> wrote:
>
> James,
> A good point.
> IUCr/PLATON/checkCIF sends out an A-level ALERT for this. Referees should have picked this up.
> Older SHELXL versions put in a default value for those values in the CIF they generated (Room Temperature)
> A quick check of the CIF's for all structures published in the 2021 issues of Acta Cryst. E77 shows that there are no inconsistencies there.
>
> Best wishes
>
> Ton
>
>
> Op 18-01-2022 06:47 heeft coreDMG namens James H via coreDMG <coredmg-bounces at iucr.org namens coredmg at iucr.org> geschreven:
>
>     Happy New Year everybody.
>
>     Here is our first issue for the year:
>
>     At first glance it would appear to me that _diffrn_ambient_temperature and
>     _cell_measurement_temperature are describing the same thing. There may
>     have been a difference in the past when using point detectors and the
>     cell was determined separately to the data collection, but for
>     anything collected on an area detector I would have expected these
>     values to be the same. However, I've noticed from looking at around
>     10000 files from the COD that, of the files that have both data names,
>     around 1% have different numbers. I'm wondering what this could
>     possibly mean, especially when the two numbers are very different e.g.
>     150K and 296K. Given that the cell parameters are necessary to obtain
>     absolute distances from fractional coordinates, if the diffraction
>     data are collected at a different temperature to the cell measurement,
>     then surely these atomic distances would be incorrect?  How do the
>     database curators handle this? How are software authors interpreting
>     these two data names for output or input?
>
>     If you agree that there is no way these numbers should be
>     significantly different, suggestions on how to fix the situation are
>     welcome. Personally I would deprecate _cell_measurement_temperature or
>     state that it is only used when the cell is measured separately to the
>     main diffraction data set.
>
>     James.
>
>     --
>     T +61 (02) 9717 9907
>     F +61 (02) 9717 3145
>     M +61 (04) 0249 4148
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-- 
T +61 (02) 9717 9907
F +61 (02) 9717 3145
M +61 (04) 0249 4148