"single species" vs. "multiple species"

[s9105947]

An openPMD speciesType can be either a single or multiple species.

Clarification in Allowed Values

Currently, how to specify multiple species is only mentioned in the description, but not under allowed values.
(imo this should be also mentioned there.)

Separator Positioning

Is a trailing separator valid, i.e. would electron;positron; be a valid species type?
And if this is allowed, are empty elements permitted too, e.g. electron;;positron;?

Clear Separation

I'd like to suggest to separate those two cases more clearly in the standard.
("A speciesType is either a single species or multiple species. Multiple species are a semicolon-separated list of single species with at least one element. A single species is either an elementary particle, a ...")

In some contexts only a single species is allowed (especially if the standard is re-used, as in PICMI).
A clear distinction makes a reference simpler, e.g. PICMI could differentiate: "At least one species as specified by the openPMD 'Convention for Specifying Particle Species'" vs. "A single species as specified by the openPMD ..."

Quantities

Would it make sense to allow a species to be prefixed with a quantity?

Obviously this is not a property of a species itself, and also would interfere with existing setups (which do not expect such a quantity).

Nevertheless that would be useful when specifying multiple species and their relative quantities, e.g. 2H^+;He^2+.

I'd be interested to hear your opinion on this.

[ax3l]

I'd like to suggest to separate those two cases more clearly in the standard.
("A speciesType is either a single species or multiple species. Multiple species are a semicolon-separated list of single species with at least one element. A single species is either an elementary particle, a ...")

Sounds good to me, please feel free to post a PR.

Would it make sense to allow a species to be prefixed with a quantity?

I would not generally introduce this yet without a specific use case. We have a notion for simple mixed ions as molecules (we could rephrase the section) for now, but have no use case for this yet. Most codes will put distinct species into distinct particle groups anyway, which makes many things easier for data processing.