All-Atom Quantum Mechanical Methodologies for One- and Two-Photon Absorption of Realistic Systems

Area of Science:

• Computational Chemistry
• Quantum Mechanics
• Spectroscopy

Background:

• Accurate prediction of molecular optical properties like one- and two-photon absorption (1PA and 2PA) is crucial for understanding biological processes.
• All-atom quantum mechanics (AQM) offers a detailed approach but has been computationally demanding for realistic systems.

Purpose of the Study:

• To assess advanced all-atom quantum mechanics (AQM) methodologies for calculating 1PA and 2PA in realistic biological systems.
• To evaluate the performance of single structure (ASQM) and dynamic structure (ADQM) QM workflows.

Main Methods:

• Development and application of simplified quantum chemistry methods, specifically sTD-DFT-xTB and dt-sTD-DFT-xTB schemes.
• Testing the ASQM scheme on proteins (bacteriorhodopsin, iLOV) to analyze 1PA and 2PA.
• Evaluating two ADQM variants (ADQM-Boltz. and ADQM-MD) for flavin mononucleotide in aqueous solution.

Main Results:

• The ASQM methodology successfully described higher-energy transitions in proteins involving π-conjugated amino acids.
• The ADQM-MD approach, using explicit solvent molecular dynamics snapshots, yielded spectra in striking agreement with experimental data.
• The ADQM-Boltz. approach, using a Boltzmann ensemble in implicit solvent, showed minimal improvement over ASQM.

Conclusions:

• AQM methodologies, particularly ADQM-MD, are powerful tools for accurate prediction of 1PA and 2PA in complex biological molecules.
• Simplified quantum chemistry schemes enable efficient computation of optical properties for large systems.
• Dynamic structure QM approaches are essential for capturing conformational effects influencing spectral properties.