Calculation of IR Spectra with a Fully Polarizable QM/MM Approach Based on Fluctuating Charges and Fluctuating Dipoles

The fully polarizable QM/MM approach based on fluctuating charges and fluctuating dipoles, named QM/FQFμ (J. Chem. Theory Comput. 2019, 15, 2233-2245), is extended to the evaluation of nuclear gradients and the calculation of IR spectra of molecular systems in condensed phase. To this end, analytical equations defining first and second energy derivatives with respect to nuclear coordinates are derived and discussed. The potentialities of the approach are shown by applying the model to the calculation of IR spectra of methlyoxirane, glycidol, and gallic acid in aqueous solution. The results are compared to the continuum QM/PCM and the polarizable QM/FQ, which is based on fluctuating charges only.