# Assessing Nonbonded Aggregates Populations: Application to the Concentration-Dependent IR O–H Band of Phenol

## TL;DR

This paper compares two methods for calculating nonbonded aggregate populations in phenol solutions and finds that molecular dynamics better predicts IR spectra trends.

## Contribution

A new graph-theory-based framework for identifying aggregate conformations and a comparative assessment of MD and quantum mechanical methods.

## Key findings

- Molecular dynamics better captures experimental IR trends due to improved entropic contributions.
- The graph-theory framework enables consistent comparison of aggregate population methods.
- The protocol provides a benchmark for evaluating intermolecular force fields.

## Abstract

In this work, we present two alternative computational
strategies
to determine the populations of nonbonded aggregates. One approach
extracts these populations from molecular dynamics (MD) simulations,
while the other employs quantum mechanical partition functions for
the most relevant minima of the multimolecular potential energy surfaces
(PESs), identified by automated conformational sampling. In both cases,
we adopt a common graph-theory-based framework, introduced in this
work, for identifying aggregate conformations, which enables a consistent
comparative assessment of both methodologies and provides insight
into the underlying approximations. We apply both strategies to investigate
phenol aggregates, up to the tetramer, at different concentrations
in phenol/carbon tetrachloride mixtures. Subsequently, we simulate
the concentration-dependent OH stretching IR region by averaging the
harmonic Infrared (IR) spectra of aggregates using the populations
predicted by each strategy. Our results indicate that the populations
extracted from MD trajectories yield OH stretching signals that closely
follow the experimental trends, outperforming the spectra from populations
obtained by systematic conformational searches. Such a better performance
of MD is attributed to a better description of the entropic contributions.
Moreover, the proposed protocol not only successfully addresses a
very challenging problem but also offers a benchmark to assess the
accuracy of the intermolecular force fields.