# Characterizing Infrared Spectra of OH–·(H2O)2 and OH–·(H2O)3 with Constrained Nuclear-Electronic Orbital Molecular Dynamics

## TL;DR

This paper uses advanced simulations to better understand the infrared spectra of OH–·(H2O)2 and OH–·(H2O)3 clusters, improving interpretation of complex spectral features.

## Contribution

The study introduces constrained nuclear-electronic orbital molecular dynamics to assign and interpret vibrational spectra with nuclear quantum effects.

## Key findings

- CNEO-MD simulations confirmed experimental assignments for OH–·(H2O)n clusters with n = 2 and 3.
- The method provided physical interpretations for previously unassigned spectral features.
- Machine-learned potentials enabled temperature-dependent simulations to capture vibrational mode coupling.

## Abstract

The vibrational spectra of OH–·(H2O)
n
clusters for small n have been well established experimentally, with fundamental
modes
largely assigned. However, clear assignment of highly anharmonic modes
and combination bands associated with strong hydrogen bonds, which
often manifest as broad spectral features, remains challenging. In
this work, we employ constrained nuclear-electronic orbital molecular
dynamics (CNEO-MD) to provide detailed peak assignments and plausible
physical interpretations for the vibrational spectra of OH–·(H2O)n clusters with n = 2 and 3. The CNEO framework incorporates nuclear quantum effects,
particularly nuclear quantum delocalization, through the underlying
effective potential energy surfaces. When combined with classical
molecular dynamics, CNEO-MD further captures coupling effects between
vibrational modes. Leveraging machine-learned potentials, we perform
a series of temperature-dependent CNEO-MD simulations and use the
resulting spectra to facilitate peak assignment. Our results largely
confirm the experimental assignments reported by Johnson and coworkers
[J. Chem. Phys.
2016, 145, 134304],
while also providing direct, physically grounded interpretations of
previously unassigned features.