Strategies for the Development of Conjugated Polymer Molecular Dynamics Force Fields Validated with Neutron and X‑ray Scattering

Conjugated polymers
(CPs) enable a wide range of lightweight, lower
cost, and flexible organic electronic devices, but a thorough understanding
of relationships between molecular structure and dynamics and electronic
performance is critical for improved device efficiencies and for new
technologies. Molecular dynamics (MD) simulations offer in
silico insight into this relationship, but their accuracy
relies on the approach used to develop the model’s parameters
or force field (FF). In this Perspective, we first review current
FFs for CPs and find that most of the models implement an arduous
reparameterization of inter-ring torsion potentials and partial charges
of classical FFs. However, there are few FFs outside of simple CP
molecules, e.g., polythiophenes, that have been developed over the
last two decades. There is also limited reparameterization of other
parameters, such as nonbonded Lennard-Jones interactions, which we
find to be directly influenced by conjugation in these materials.
We further provide a discussion on experimental validation of MD FFs,
with emphasis on neutron and X-ray scattering. We define multiple
ways in which various scattering methods can be directly compared
to results of MD simulations, providing a powerful experimental validation
metric of local structure and dynamics at relevant length and time
scales to charge transport mechanisms in CPs. Finally, we offer a
perspective on the use of neutron scattering with machine learning
to enable high-throughput parametrization of accurate and experimentally
validated CP FFs enabled not only by the ongoing advancements in computational
chemistry, data science, and high-performance computing but also using
oligomers as proxies for longer polymer chains during FF development.