Extensive Benchmark Study of the Resonance Raman Spectra of Lumiflavin

An extensive computational TDDFT resonance Raman study is presented here including forty-two different DFT functionals. The functionals were checked against the experimental FSRS Evolution Associated Spectrum of the equilibrated S1 state of FMN published earlier. Off-resonance spectra were computed first and yielded adequate agreement with the experimental spectrum. Fine-tuning of the correlation was achieved with the inclusion of specific scaling factors for each DFT functional, aimed to align the highest computed peak (symmetric C=O stretch) to the corresponding experimental peak in the fingerprint region. Subsequently, resonance Raman intensities were calculated with a careful choice of the resonant states. The experimental Evolution Associated Spectrum utilized for the comparisons was taken under resonance conditions, hence the inclusion of resonance enhancements in the calculations improved the agreement in most of the DFT functionals. For six particular DFT functionals, namely HCTH/407, OLYP, OPBE, O3LYP, tHCTHhyb and TPSSh, the theoretical/experimental correlation was particularly facile, and their agreement was predicted superior to the other functionals. The narrowing down to the above set was achieved by the evaluation of all DFT functionals according to five criteria ranging from the percent error of the main flavin excitations to the experimental values, to visual inspection of the spectra, and the determination of whether the inclusion of resonance in the calculation improved the agreement with experiment. Owing to the extent of the data set, valuable insights were gained to assist similar studies.