Spin assigned group labels

Separability between electronic and nuclear states is fundamental to get a description in terms of a hierarchy of electronic and subsidiary nuclear quantum numbers. Physical quantum states, i.e. wavefiinctions 0(q,Q), are non-separable. On the contrary, there is a special base set of functions Pjt(q,Q) that can be separable in a well defined mode, and used to represent quantum states as linear superpositions over the base of separable molecular states. For the electronic part, the symmetric group offers a way to assign quantum numbers in terms of irreducible representations [17]. Space base functions can hence be either symmetric or anti-symmetric to odd label permutations. The spin part can be treated in a similar fashion [17]. The concept of molecular species can be introduced this is done at a later stage [10]. Molecular states and molecular species are not the same things. The latter belong to classical chemistry, the former are base function in molecular Hilbert space. 

Flavodoxins have a natural spin label, but, unlike the nitroxides, the flavin-free radical suffers from electron de-localisation, which interferes with distance calculations. The broadening of protein resonances in the spectra of the semiquinone forms of flavodoxins from Clostridium MP and Peptostreptococcus elsdenii is highly selective [99], and peaks from groups in the vicinity of the radical can be assigned. 

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