Anharmonicity modeling in hydrogen bonded solvent dimers

Harmonic and anharmonic frequencies of dimers and mixed dimers of water, methanol and benzene were computed and the results were critically analysed to investigate the anharmonicity of the normal mode vibrations within density functional theory (DFT) with empirically included Grimme correction for dispersion (D3). From several options, the B3LYP-D3/6-31++G* level of theory was selected as a good compromise between accuracy and calculation speed, suitable for future modeling of larger solvent clusters. The obtained raw harmonic and anharmonic second-order perturbation theory of vibrational frequencies (VPT2) were additionally scaled using a two-range procedure (below and above 2000 cm ⁻¹) and compared with available experimental values for the studied dimers. Additionally, the Morse and 4th order polynomial fittings of small normal mode deflections from the equilibrium geometry were used to model anharmonicity of the studied dimers. Compared to B3LYP-D3, the VPT2 vibrations calculated with APFD and M06-2X functionals very poorly model the anharmonicity of the H-bonded dimers. The surprising observation was related to the performance of CCSD/6-31++G* and CCSD(T)/6-31++G* in predicting harmonic frequencies of H-bonded dimers of water, water-methanol and methanol-water of similar quality, as compared to B3LYP-D3/6-31++G*. The obtained results suggest using of B3LYP-D3/6-31++G* scaled harmonic frequencies for the larger solvent clusters as a good alternative to anharmonic frequencies.