Discussion: Units

[adrybakov]

Topic: physical units for the data in the format.

[adrybakov]

I propose to use the pair "key - value" as a part of the files metadata for the units. This type of data structure is probably supported in any format that can be chosen as a parent format (see #4).

For the parameters of the Hamiltonian the units are energy or energy-like. I propose to support a set of usual units by keyword and a custom unit vial conversion constant to the SI's Joule.

Here is an example of how this can be implemented, using abstract format:

For key-word supported units

# key : value
parameters_units : meV

Supported keywords (case-insensitive): "meV", "eV", "Kelvin", "Joule", "Rydberg", "Erg"

For custom units

# key : value
parameters_units : custom
parameters_units_constant : 1.986445e-23

Edit: Fix a typo

[adrybakov]

If the crystal structure will be included in the file (see #5), then the units of length should be discussed as well.

One approach is to leave it unspecified.

Another one is to treat them in the same way as energy units, using meter as a reference for the custom unit

# key : value
length_units : Angstrom

# key : value
length_units : custom
length_units_constant : 1e-10

[lucas-wilkins]

Suggestions:

1. keep the list of allowed units minimal
2. if reasonable, disallow all units of incompatible dimensionality, for example, no using wavenumber as an energy unit.
3. have a single allowable representation for each unit, so only one of "A", "a", "Ang", "AU", "au", "A.U.", "a.u.", "Å", the other "Å" that looks the same but is actually two letters, the deprecated unicode "Å" (U+212B), "ang", "Angstrom", "Angstroms", "Ångström", "Ångströms", "ångström", "ångströms", "Ångström", "Ångströms", "angström", "angströms", "Ångström", "Ångströms", "ångstrom", "ångstroms"... I could go on.

[adrybakov]

Wow, this is an impressive amount of Angstroms.

Fully agree with the point 3.

The points 1 and 2 collide a bit with the idea of the support for custom unit. @lucas-wilkins, could you expand you view on the support for arbitrary custom units, please?