The NTO representation (Appendix B) is now widely used for interpretative purposes. NTOs are the best oneparticle orbitals in the well-defined sense of minimizing configuration mixing by reducing the transition density to a minimal number of non-vanishing amplitudes, λ i (i = 1, . . . , n occ ), thus minimizing configuration mixing. Although sometimes defined using singular value decomposition of x, 97-100 a more general definition when y = 0 is that the NTOs diagonalize both ∆P elec and ∆P hole . For future reference, note that the particle and hole densities in the NTO basis are