can you show your molecule? In some cases, band assignment made by experimentalists is simply incorrect. In other cases, it is TDDFT that misplaces the transitions of different nature (n-pi, pi-pi*, or charge-transfer). You can look at the effect of HF exchange in the functional by using the following series: BLYP (0% HF exchange) - B3LYP (20%) - BHHLYP (50%) - LYP (100%). If the effect is strong, CASSCF-XMCQDPT can be the method of choice. An alternative way is to use range-separated hybrid functionals, such as LC-BLYP or CAM-B3LYP (implemented in GAMESS-US), although the results can still be questionable.
On Sun Mar 30 '14 3:36pm, Alisher wrote
>Dear Alex and Firefly users,
>I computed UV/Vis spectra of C=O containing aromatic compounds by TDDFT (B3LYP1). But, I canít found agreement between experiment and TDDFT. In the experiment, usually, n-pi* transition of the C=O group observed in the longest wavelength region then pi-pi* transition. Besides, n-pi and pi-pi* transitions also observed pi-pi* transition caused by charge transfer from aromatic fragment to the C=O group.
>Who can help me to choice the best DFT method for modeling UV/Vis?
>Thanks in advance!