Two-Photon Absorption Strengths of Small Molecules: Reference CC3 Values and Benchmarks

Area of Science:

• Computational chemistry
• Molecular spectroscopy
• Quantum mechanics

Background:

• Accurate calculation of molecular properties is crucial for understanding chemical phenomena.
• Two-photon absorption (TPA) is a significant photophysical process with applications in various fields.
• High-accuracy computational methods are needed to reliably predict TPA strengths.

Purpose of the Study:

• To create a comprehensive dataset of highly accurate two-photon transition strengths for standard small molecules.
• To provide a benchmark for evaluating the performance of various computational methods for TPA calculations.
• To assess the impact of system size, transition character, and computational approximations on prediction accuracy.

Main Methods:

• Reference TPA strengths calculated using the quadratic response implementation of the third-order coupled cluster method including iterative triples (Q-CC3).
• Aug-cc-pVTZ and aug-cc-pVDZ atomic basis sets were employed, with basis set effects discussed.
• Evaluated alternative wavefunction methods: quadratic response and equation-of-motion CCSD approximations, Q-CC2, and I-ADC2.
• Assessed time-dependent density functional theory (TD-DFT) using five common exchange-correlation functionals.

Main Results:

• A dataset of 82 singlet transitions (Rydberg, valence, double excitations) was generated.
• Quantitative assessment of benchmarked methods revealed varying performance based on system size, transition intensity, and type.
• Insights into the accuracy and limitations of different computational approaches for predicting TPA strengths were obtained.

Conclusions:

• The Q-CC3 method provides highly accurate reference values for TPA strengths.
• The developed dataset serves as a valuable resource for validating and improving computational methods.
• Understanding method performance is key for selecting appropriate computational strategies for TPA calculations.