All-atom quantum mechanical (AQM) methodologies for one- and two-photon absorption of realistic systems.

All-atom quantum mechanics (AQM) methodologies are assessed to evaluate one- and two-photon absorption (1PA and 2PA) of realistic systems. All-atom single structure QM (ASQM) and dynamic structure QM (ADQM) methodologies are discussed. These workflows are possible thanks to developments in simplified quantum chemistry methods and in particular with both sTD-DFT-xTB and dt-sTD-DFT-xTB schemes. The ASQM scheme is tested to compute the 1- and 2PA of two proteins: bacteriorhodopsin and iLOV. Results show that the ASQM methodology is able to describe higher-energy transitions involving π-conjugated amino acids such as tryptophan or tyrosine. Then, two variants of the ADQM workflow are evaluated to reproduce the 1- and 2PA of the flavin mononucleotide (FMN) in aqueous solution, involving either Boltzmann ensemble of conformers in implicit solvent (ADQM-Boltz.) or snapshots of molecular dynamics of explicitly solvated systems (ADQM-MD). Spectra computed with the ADQM-MD approach provide striking comparisons with respect to experiment while the ADQM-Boltz. approach provides little change with respect to the ASQM workflow.