I have this problem quite often, where I'm computing a subset of SOAP vectors for who species ABCD are present, then another subset where only ABD are present. I know that I can use the species pair dictionary to collect SOAPs per species pair, but it would be far, far more convenient to do something like this:

hyperparameters = {
    "soap_type": "PowerSpectrum",
    ...

    # I know this keyword exists, but seems to only affect `expansion_by_species_method`
    "global_species": [A, B, C, D, ...] 
}
soap = SOAP(**hyperparameters)

featuresABCD = soap.transform(framesABCD).get_features(soap)
featuresABD = soap.transform(framesABD).get_features(soap)

where featuresABCD and featuresABD have the same indexing and shape.