Metrics for Molecular Electronic Excitations: A Comparison between Orbital- and Density-Based Descriptors

This study proposes a quantitative and qualitative comparison of two popular metrics used for time-dependent density functional simulations of chromophores when describing absorption and emission processes, with high discrimination power between short- and long-range character of involved electronic excitations and functional performances. To this end, a total of 160 absorption and emission electronic excitations of 80 molecular systems belonging to the "Real-Life Molecules" data set, recently introduced in literature, have been considered a relevant data set. The two selected indexes are based on density (the D-CT one) and natural transition orbitals (the Delta r(NTO) one), respectively. For comparison purposes, an extension of the D-CT index, in line with what exists for Delta r(NTO), enabling to discriminate electronic transitions occurring in syminetric systems is also proposed. The results show that, independently of the exchange and correlation functional used, a good correlation between the natural transition orbital and density based descriptors is found, thus cross validating their use for the quantification of a large variety of transitions in chemically relevant molecular systems.