# Atomistic QM/Classical Modeling of Surface-Enhanced Infrared Absorption

## TL;DR

This paper introduces a new computational method to model infrared absorption of molecules on nanostructures, using quantum and classical approaches for accuracy and efficiency.

## Contribution

A multiscale QM/MM approach combining DFT with frequency-dependent atomistic models for plasmonic materials is proposed.

## Key findings

- The method accurately models plasmonic responses of graphene and noble metals.
- SEIRA spectra of adenine on gold nanoparticles and graphene were successfully simulated.
- Results match well with experimental data and SERS spectra.

## Abstract

We present a multiscale quantum mechanics/classical (QM/MM)
approach
for modeling surface-enhanced infrared absorption (SEIRA) spectra
of molecules adsorbed on plasmonic nanostructures. The molecular subsystem
is described at the density functional theory (DFT) level, while the
plasmonic material is represented using fully atomistic, frequency-dependent
Fluctuating Charges (ωFQ) and Fluctuating Charges and Dipoles
(ωFQFμ) models. These schemes enable an accurate and computationally
efficient description of the plasmonic response of both graphene-based
materials and noble metal nanostructures, achieving accuracy comparable
to that of ab initio methods. The proposed methodology
is applied to the calculation of SEIRA spectra of adenine adsorbed
on gold nanoparticles and graphene sheets. The quality and robustness
of the approach are assessed through comparison with surface-enhanced
Raman scattering (SERS) spectra and available experimental data. The
results demonstrate that the proposed framework provides a reliable
route to simulate vibrational responses of plasmon–molecule
hybrid systems.