Diagonalizing the 1-TDM matrix gives eigenvectors that represent the hole and particle natural transition orbitals (NTOs). The square of the norm of the 1-TDM matrix, Ω, defined in equation 1, provides a numerical indicator of the single-excitation character of the transition. For quantum chemistry methods that include single-excitations only, such as TDDFT and configuration interaction singles(CIS), Ω=1 . But for quantum chemistry methods that include single and higher excitations, for most transitions the norm (Ω EOM -CCSD ) is less than 1; larger deviations from this quantity indicate a state with more contribution from double and higher excitations.