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I have just received an error when using the siunitx package that the angström length unit has been deprecated. Since when? and more importantly WHY?

Has anyone seen this error? Now in order to use that unit one has to define it.

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You don't have to define it, but I recommend it as it's now a non-standard unit. The reason it's been deprecated is that the BIPM have removed it from the SI Brochure. As a matter of policy, I only predefine 'official' units - there are simply too many other units that people could argue for that I'd have to make a judgement on.

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    but why?! we use it all the time in materials science. In any case, I had to define it or I would get an error when using vimtex.
    – lucian
    Aug 11 at 12:51
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    @lucian It's a warning not an error, so it should not be a major issue. As to why, I think since ångström are similar a power of ten of metres, the BIPM feel we should all just use picometres.
    – Joseph Wright
    Aug 11 at 13:14
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    @frougon I guess you could go ahead and make a package called something like fix-siunitx where you define all the units you find to be missing. A lot of people might even appreciate it if you do.
    – Gaussler
    Aug 11 at 13:31
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    @Gaussler After a bit of discussion, I've kept macros for almost of the v2 units in v3, it's just that they issue a warning. I think that's a reasonable balance: I have a line, I'm sticking to it as close as I can without breaking existing documents unnecessarily.
    – Joseph Wright
    Aug 11 at 13:34
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    @frougon I love long option names. Next, we need something like enable-that-ugly,font-that-will-annoy-egreg-like-hell (activates Comic Sans). ;-)
    – Gaussler
    Aug 11 at 14:13
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You can define the unit as

\DeclareSIUnit\angstrom{\text{\AA}}

In this way you can use the unit without a warning in compilation.

I can't understand why these units were deprecated... Well, they do not belong to SI, but lots of them are in use by the scientific community. I'm having lots of troubles with the new version of siunitx (Debian Sid updated Texlive a couple of days ago); for example, now you need to define

\sisetup{input-digits=0123456789\pi/}

in order to write

\num{\pi/3}
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    I deprecated \angstrom as the BIPM have - they set the rules for the SI.
    – Joseph Wright
    Oct 1 at 9:33
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    On \pi, well it's not a number, is it - you can't measure \pi or something, only 3.1415 \pm (some error)
    – Joseph Wright
    Oct 1 at 9:33

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