Rewriting this in terms of S∆P, as in Eqs. (4.1) and (4.2), suggests an atomic partition analogous to Mayer's bond-order matrix, M. For a closed-shell system, that quantity has matrix elements The quantity ∆M AB can then be taken as an alternative definition of a CT number, 186 a convention that has since been adopted by others. Alternatively, one might exploit trace invariance in tr(∆M) =