Benchmarking two-photon absorption with CC3 quadratic response theory, and comparison with density-functional...

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Molecular Spectroscopy

Background:

• Two-photon absorption (TPA) is crucial for understanding molecular properties and interactions.
• Accurate theoretical calculations of TPA cross sections are essential for experimental validation and prediction.
• Electron correlation plays a vital role in determining the electronic transitions involved in TPA.

Purpose of the Study:

• To investigate the influence of electron correlation on two-photon absorption cross sections.
• To evaluate the performance of various coupled cluster (CC) models and density-functional theory (DFT) functionals for TPA calculations.
• To determine the effect of triple excitations on TPA cross sections for the first time.

Main Methods:

• Coupled cluster quadratic response theory was employed.
• A hierarchy of CC models (CCS, CC2, CCSD, CC3) was utilized.
• Hartree-Fock (HF) and DFT response theories were used for comparison, including LDA, BLYP, B3LYP, and CAM-B3LYP functionals.
• Calculations were performed for formaldehyde (CH2O), diacetylene (C4H2), and water (H2O).

Main Results:

• Electron correlation significantly affects two-photon absorption cross sections.
• Coupled cluster methods, especially those including triple excitations (CC3), provide accurate results.
• The CAM-B3LYP functional shows promise for TPA calculations, particularly with diffuse basis sets, but requires caution for Rydberg states.
• Significant differences were observed between CC and DFT results, highlighting the importance of electron correlation treatment.

Conclusions:

• Coupled cluster theory, accounting for electron correlation and triple excitations, is a reliable method for calculating two-photon absorption cross sections.
• The CAM-B3LYP functional is a promising DFT approach for TPA, but its performance for diffuse states needs further investigation.
• Accurate theoretical predictions of TPA are crucial for advancing molecular science and spectroscopy.