Ultrafast Evaluation of Two-Photon Absorption with Simplified Time-Dependent Density Functional Theory

Area of Science:

• Computational Chemistry
• Quantum Mechanics
• Spectroscopy

Background:

• Two-photon absorption (2PA) is crucial for advanced optical applications.
• Accurate theoretical evaluation of 2PA cross-sections is computationally demanding for large systems.
• Simplified time-dependent density functional theory (sTD-DFT) offers a computationally efficient alternative.

Purpose of the Study:

• To develop and validate a new implementation of sTD-DFT for ultrafast 2PA cross-section calculations.
• To assess the accuracy of the method for various molecular systems, including large biomolecules.
• To enable theoretical prediction of 2PA spectra for complex systems relevant to fluorescent proteins and other applications.

Main Methods:

• Utilized a novel sTD-DFT implementation based on regular DFT ground-state determinants.
• Employed a tight-binding sTD-DFT-xTX variant for very large molecular systems (up to ~2000 atoms).
• Benchmarked the method against higher-level quantum chemistry calculations and experimental 2PA spectra.

Main Results:

• Achieved ultrafast evaluation of 2PA cross-sections for large molecules.
• Demonstrated striking agreement between calculated and experimental 2PA spectra for eGFP, flavin mononucleotide, and iLOV protein.
• Successfully extended the all-atom quantum chemistry methodology to systems of approximately 2000 atoms.

Conclusions:

• The sTD-DFT approach provides a computationally efficient and accurate tool for predicting 2PA cross-sections.
• This methodology is suitable for analyzing complex biological chromophores and large molecular systems.
• The developed method significantly advances the theoretical study of two-photon absorption phenomena.