On the maximum number of 'odd' electrons in a molecule

The definition of 'odd' electrons of Yamaguchi et al. and the related notion of 'effectively unpaired electrons' are revisited. An explanation is suggested for the bias with these definitions discussed recently by Head-Gordon. It is shown that some of the confusion is due to misapprehension of the spinless odd electron density in the context of the single-reference methods used. Starting from a more general spin-dependent formulation seems to put things into places.