[ase-users] Backwards-compatible change to how VASP chooses default pseudopotentials

[Eric Hermes]

Hello ASE+VASP users,

Thanks to the work of Emmanuel Farhi, the ASE VASP calculator in git
now chooses a default pseudopotential for those elements that do not
have a suffix-free option (for example, there is no "Ca"
pseudopotential). In those cases, the calculator will now default to
the semicore pseudopotential that includes the fewest number of core
electrons in the valence. The only effect of this change is that
scripts which previously would raise a RuntimeError (e.g. 'No
pseudopotentials for Ca!') now will be able to find a pseudopotential.
The behavior of previously working scripts should not change! You can
override the default behavior with the 'setups' keyword, just as
before.

If, for whatever reason, your VASP pseudopotential directory has
suffix-free pseudopotentials for elements that lack suffix-free options
in the official VASP pseudopotential library, this might change the
behavior of your scripts! If this affects you, then you should change
your custom pseudopotentials to have a custom suffix, and pass that
suffix through the 'setups' keyword (e.g. setups={'Ca': '_custom'}).

Eric