Several methods are available to compute the anharmonicity in semi-rigid molecules. However, such methods are not routinely employed yet because of their large computational cost, especially for large molecules. The potential energy surface is required and generally approximated by a quartic force field potential based on ab initio calculation, thus limiting this approach to medium-sized molecules. We developed a new, fast and accurate hybrid Quantum Mechanic/Machine learning (QM//ML) approach to reduce the computational time for large systems. With this novel approach, we evaluated anharmonic frequencies of 37 molecules thus covering a broad range of vibrational modes and chemical environments. The obtained fundamental frequencies reproduce results obtained using B2PLYP/def2tzvpp with a root-mean-square deviation (RMSD) of 21 cm −1 and experimental results with a RMSD of 23 cm −1 . Along with this very good accuracy, the computational time with our hybrid QM//ML approach scales linearly with N while the traditional full ab initio method scales as N 2 , where N is the number of atoms.
Using LR-TDDFT, we calculated the 0-0 energies of 15 small radicals for which the experimental values in gas phase are available. We used 17 functionals. It turned out that B3LYP, M06-2X, ωB97X-D, CAM-B3LYP, and HSE06 functionals are the most effective functionals in terms of root-mean-square and average unsigned deviation. Using the standard value (0.47 a0(-1)) of the attenuation parameter ω, the long-range-corrected LC-GGA functionals give poor results. However, the LC-PBE with ω = 0.25 a0(-1) give a performance similar to that of B3LYP. Taking into account zero-point correction improves the results, but the contribution of adiabatic correction is more important than that due to the vibration. The vertical approximation is certainly not recommended. An adiabatic calculation seems to give a good compromise between computing time (and resources) and reliability of the results for most of molecules investigated in this work.
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