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

## TL;DR

This paper extends a fully polarizable QM/MM approach to compute nuclear gradients and IR spectra, demonstrating its effectiveness on various molecules in aqueous solution and comparing it with other models.

## Contribution

It introduces analytical equations for energy derivatives in the QM/FQFμ approach and applies them to calculate IR spectra in condensed phases.

## Key findings

- Accurate IR spectra of molecules in solution were obtained.
- The approach outperforms or matches existing models like QM/PCM and QM/FQ.
- Analytical derivatives enable efficient spectrum calculations.

## Abstract

The fully polarizable QM/MM approach based on fluctuating charges and fluctuating dipoles, named QM/FQF{\mu} (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 with the continuum QM/PCM and the polarizable QM/FQ, which is based on Fluctuating Charges only.