e-h vanishes in centrosymmetric systems, as does its doppelgänger, ∆r. This is a significant drawback in some cases, but which is not shared by d CD . Alternatively, to obtain a non-vanishing metric in the presence of inversion symmetry, an "effective electron displacement" measure has been suggested, defined as Γ = ∆r + ∆σ . The quantity σ 2 r is the second moment of orbital ψ r . In a sense, ∆σ is conceptually similar to d exc in eqn. (5.12) in the same way that ∆r is conceptually similar to d - e-h , with the important distinction that both ∆r and ∆σ mangle the normalization when y is nonzero, and that the numerical values of both ∆r and ∆σ depend upon the choice of representation.