Equation (3.3) demonstrates how the NTOs extract the most important contributions to the particle and hole densities, or in other words the most significant contributions to the unrelaxed attachment and detachment densities. In the case where there is only one significant singular value (λ 2 1 ≈ 1), then |ψ elec 1 (r)| 2 and |ψ elec 1 (r)| 2 are equivalent to the particle and hole densities ∆ρ elec (r) and ∆ρ hole (r), respectively, or in other words they are equivalent to unrelaxed attachment and detachment densities. This connection does not seem to be widely appreciated.