Excited-State Absorption: Reference Oscillator Strengths, Wave Function, and TDDFT Benchmarks

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Spectroscopy

Background:

• Excited-state absorption (ESA) is crucial for understanding light-matter interactions and interpreting time-resolved experiments.
• Accurate computational modeling of ESA is essential for theoretical and experimental chemists.

Purpose of the Study:

• To create a dataset of ESA oscillator strengths and vertical transition energies for 71 excited states across 21 molecules.
• To evaluate the performance of various computational methods, including quadratic response coupled cluster (QR-CC3), quadratic response time-dependent density functional theory (QR-TDDFT), and other wave function methods, for ESA calculations.
• To investigate the impact of geometry relaxation on excited-state calculations.

Main Methods:

• Calculated 53 ESA oscillator strengths and vertical transition energies using the quadratic response (QR) CC3 formalism with eight Dunning basis sets.
• Assessed the performance of QR-TDDFT (with and without Tamm-Dancoff approximation) using various functionals (B3LYP, BH&HLYP, CAM-B3LYP, LC-BLYP33, LC-BLYP47).
• Compared QR-CC3 results with lower-order wave function methods (QR-CCSD, QR-CC2, EOM-CCSD, ISR-ADC(2), ISR-ADC(3)) and analyzed the effect of excited-state geometry relaxation.

Main Results:

• The d-aug-cc-pVTZ basis set is adequate for reference calculations, while d-aug-cc-pVDZ performs well in most cases.
• QR-TDDFT methods show acceptable errors for ESA oscillator strengths, with CAM-B3LYP demonstrating particular promise, especially for larger molecules and in the Franck-Condon region.
• ISR-ADC(3) performs excellently in the Franck-Condon region. The accuracy of TDDFT functionals for ESA calculations is sensitive to the chosen molecular geometry.

Conclusions:

• The developed dataset and method comparisons provide valuable benchmarks for future theoretical studies of excited-state absorption.
• CAM-B3LYP and ISR-ADC(3) emerge as promising methods for accurate ESA calculations, particularly in the Franck-Condon region.
• The choice of exchange-correlation functional in TDDFT significantly impacts accuracy when using relaxed excited-state geometries.