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

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Spectroscopy

Background:

• Accurate simulation of molecular absorption and emission is crucial for understanding chromophore behavior.
• Existing metrics for time-dependent density functional theory (TD-DFT) simulations have limitations in discriminating electronic excitation character.

Purpose of the Study:

• To quantitatively and qualitatively compare two popular TD-DFT metrics: D_CT (density-based) and Delta r_NTO (natural transition orbital-based).
• To evaluate their effectiveness in describing absorption and emission processes, particularly for distinguishing short- and long-range electronic excitations.
• To introduce an extension of the D_CT index for symmetric systems.

Main Methods:

• Utilized a dataset of 160 absorption and emission electronic excitations from 80 "Real-Life Molecules".
• Performed quantitative and qualitative comparisons of D_CT and Delta r_NTO metrics.
• Applied an extended D_CT index to handle symmetric molecular systems.

Main Results:

• A strong correlation was observed between the density-based D_CT and natural transition orbital-based Delta r_NTO descriptors.
• This correlation held true regardless of the specific exchange and correlation functional employed in the TD-DFT simulations.
• Both metrics demonstrated high discrimination power for various electronic transition types.

Conclusions:

• The D_CT and Delta r_NTO metrics are cross-validated and reliable for quantifying electronic transitions in diverse molecular systems.
• These validated metrics enhance the predictive power of TD-DFT for chromophore simulations.
• The findings support the robust application of these descriptors in computational chemistry research.