Evaluating the Tamm-Dancoff Approximation for Two-Photon Absorption Properties.

Area of Science:

• Quantum Chemistry
• Nonlinear Optics
• Computational Photophysics

Background:

• Two-photon absorption (2PA) is crucial for advanced applications like microscopy and photodynamic therapy.
• Accurate prediction of 2PA properties is essential for designing novel materials and optimizing existing technologies.
• The computational cost and accuracy of different theoretical methods for 2PA property prediction require further investigation.

Purpose of the Study:

• To evaluate the performance of time-dependent density-functional theory with the Tamm-Dancoff approximation (TDA) for predicting 2PA properties.
• To compare TDA predictions with full time-dependent density-functional theory (TD-DFT) and resolution-of-identity coupled-cluster (RI-CC2) methods.
• To assess the accuracy of various functionals (CAM-B3LYP, MN15, ωB97X) within TDA and TD-DFT frameworks.

Main Methods:

• Utilized time-dependent density-functional theory (TD-DFT) and TD-DFT with the Tamm-Dancoff approximation (TDA) for 20 chromophores.
• Employed the resolution-of-identity approximation with second-order approximate coupled-cluster (RI-CC2) as a benchmark.
• Calculated key photophysical properties: 2PA cross sections (σ^2PA) and dipole moments (μ₀₀, μ₁₁, μ₀₁, Δμ).

Main Results:

• The MN15 functional exhibited the lowest mean absolute errors (MAEs) for both TDA and TD-DFT calculations.
• TDA successfully captured qualitative trends in 2PA properties.
• TDA provided slightly more accurate quantitative predictions with generally lower MAEs compared to TD-DFT.

Conclusions:

• The Tamm-Dancoff approximation (TDA) is a promising, cost-effective alternative to full TD-DFT for predicting 2PA properties.
• TDA offers a favorable balance between computational efficiency and predictive accuracy for 2PA calculations.
• Further studies validating TDA's performance across a wider range of chromophores and functionals are warranted.