Pitfall in simulations of vibronic TD-DFT spectra: diagnosis and assessment

In this Communication, we study the effect of spurious oscillations in the profiles of energy derivatives with respect to nuclear coordinates calculated with density functional approximations (DFAs) for formaldehyde, pyridine, and furan in their ground and electronic excited states. These spurious oscillations, which can only be removed using extensive integration grids that increase enormously the CPU cost of DFA calculations, are significant in the case of third- and fourth-order energy derivatives of the ground and excited states computed by M06-2X and omega B97X functionals. The errors in question propagate to anharmonic vibronic spectra computed under the Franck-Condon approximation, i.e., positions and intensities of vibronic transitions are affected to a large extent (shifts as significant as hundreds of cm-1 were observed). On the other hand, the LC-BLYP and CAM-B3LYP functionals show a much less pronounced effect due to spurious oscillations. Based on the results presented herein, we recommend either LC-BLYP or CAM-B3LYP with integration grids (250, 974) (or larger) for numerically stable simulations of vibronic spectra including anharmonic effects. The effect of spurious oscillations in geometrical energy derivatives computed using density functional approximations is studied for the ground and electronic excited states.

Automated summary

In this study, the effect of spurious oscillations in energy derivatives calculated using density functional approximations (DFAs) for different molecules and states is investigated. It is found that these oscillations can only be removed by using extensive integration grids at the expense of increased computational cost. The errors in the derivatives significantly affect the anharmonic vibronic spectra, leading to shifts in the transition positions and intensities. LC-BLYP and CAM-B3LYP functionals show less pronounced effects compared to other functionals.