hbar in constants about qcelemental HOT 4 OPEN

* The aliases that are there grandfathered from psi4. I don't object to more (and hbar is a good candidate) but I'd like to gather opinions about whether we should be free or sparing with them.

I would prefer to be free. I think it's in the spirit of qcel to have as much chemistry (meta)data as possible.

* Associated is that I aliased the codata 2014 constant names into 2018. how far to we wwant to continue this? 2014 to 2022 or just 2018 to 2022

I think we can keep doing this going forward. We only have to do it once every 4 years, and surely they don't change that many names...

* Note that the constants haven't been systematically fed into pint. there's some it may not recognize:  GeV, T (period)

I propose we feed the constants into pint (with those constants whose units are not supported being silently ignored [or warn at test-time])?

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I am for more aliases, the only possible downside here is that we contaminate the space too much making it hard to use. I think this may be hard to do (would need ~5-10x the constants we have now).

As a note on items like GeV, pint understands prefixes:

>>> qcel.constants.ureg("GeV")
<Quantity(1, 'gigaelectron_volt')>

We might want to explain the Hz thing in the speed of light as it is a bit awkward in code comments.

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Moving further comments here:

@loriab:

I suspect that most qcel clients are comfortable with direct physical constants and conversion_factor() seems newfangled. So ureg gets most use as user API and isn't so fully built out. I think we want to keep context and ureg separate as they have different sources.

@dgasmith

To refine, I think we want to keep the PhysicalConstantsContext class, but slowly deprecated the aliases in favor of direct ureg generated ones?

This would be my vote anyways so we have a single source of truth.

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To refine, I think we want to keep the PhysicalConstantsContext class, but slowly deprecated the aliases in favor of direct ureg generated ones?

This would be my vote anyways so we have a single source of truth.

I like the idea, but my reservations are:

• surely pint doesn't like units with spaces. I rather like the idea that a project can use straight-from-codata names through qcel w/o qcel name embellishment.s
• the PhsysicalConstantsContext class doesn't just supply conversion_factor(), for which ureg can take over, but also compiled headers. This is true for psi at least and I imagine other codes that might adopt qcel will be slower still to change their physconst names or move all from-au conversions out of compiled code.

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