Real and imaginary excitons: Making sense of resonance wavefunctions by using reduced state and transition density matrices
An analysis scheme of real and imaginary parts of the resonance wavefunctions by using reduced transition density matrices and natural transition orbitals is presented. While real parts of excitons describe changes in the electron density corresponding to the bound part of the resonance, the imaginary excitons can be interpreted as virtual states facilitating one-electron decay into the continuum. Different nature of the real and imaginary excitons is revealed by exciton descriptors, in particular hole-particle separation and their correlation. Singular values and respective participation ratios quantify the extent of collectivity of the excitation and a number of distinct decay channels.