y Structure and vibrational spectra of p-coumaric acid dimers by DFT methods

We performed a systematic investigation to determine the most accurate DFT functional in the prediction of geometrical parameters and vibrational frequencies for p-coumaric acid (p-CA). For this purpose, we followed a multi-dimensional strategy by performing calculations with 8 DFT functionals for the p-CA dimer with fine and ultrafine grids. We also compared scaled and unscaled harmonic frequencies of p-CA with experimental ones. The unscaled BP86 harmonic frequencies were the most accurate relative to the experimental ones but the BP86 method had the lowest accuracy for the structural parameters. The dispersion-corrected method, wB97XD, showed the best performance for the geometries of the p-CA dimer, though it yielded the highest RMSDs for the vibrational frequencies. We observed that the methods PBE0/def2-TZVP and M06-2X/def2-TZVP, with an ultrafine grid, provided a cost-effective approach which predicted accurately both geometrical parameters and scaled vibrational frequencies of p-CA.