Assessment of TD-DFT and CC2 Methods for the Calculation of Resonance Raman Intensities: Application to o-Nitrophenol

The resonance Raman (RR) intensities of o-nitrophenol (oNP) were investigated theoretically with the aim of assessing the accuracy of excited state gradients calculated with DFT and CC2 approaches. It is found that the B3LYP and B2PLYP exchange-correlation (XC) functionals provide the best estimate of the ground state properties, while the other considered approaches present significantly less accurate vibrational frequencies and normal coordinates. Then, it is demonstrated that the use of the B3LYP force field for the ground state properties, in association with XC functionals including a large amount of HF exchange (M06-2X) or including long-range corrections (CAM-B3LYP and omega B97X) for the excited state gradient calculations, provides the most accurate RR spectra. Moreover, it is found that the RR intensities calculated with the best XC functionals show comparable accuracy to the results obtained with CC2 calculations. Finally, it is seen that the accuracy of the excited state gradients does not correlate with the accuracy of the excitation energies and oscillator strengths, for which XC functionals with a lesser amount of HF exchange (B3LYP, M06, and HSE06) provide more accurate results in the case of oNP. This indicates that the assessment of excited state gradients via the calculation of RR intensities, can provide additional information about the performance of quantum chemistry approaches in predicting excited state properties.