Polaritonic Chemistry

Polaritonic chemistry is an emerging field that explores how chemical reactions and molecular properties can be modified by strong coupling between molecules and confined light fields in optical cavities. When molecules are placed between two highly reflective mirrors forming a cavity, their electronic or vibrational transitions can strongly interact with the cavity’s electromagnetic field modes. This interaction leads to the formation of hybrid light-matter states called polaritons. The field combines concepts from quantum optics, physical chemistry, and materials science.

A schematic showing how molecules in an optical cavity can couple with confined light to form polaritons - hybrid light-matter states that can modify chemical reactivity. Figure adapted from J Yuen-Zhou and VM Menon, PNAS (2019)

The formation of polaritons can dramatically alter the energy landscape and dynamics of chemical systems, enabling:

• Modification of reaction rates and pathways
• Control over energy transfer processes
• Enhanced charge separation in photovoltaic materials
• Development of novel chemical sensors
• Creation of quantum materials with unique properties

My research focuses on developing theoretical frameworks and computational methods to understand and predict how strong light-matter coupling affects chemical processes at both the microscopic quantum level and macroscopic ensemble level. All works are in collaboration with Prof. David R. Reichman, Prof. Arkajit Mandal, and Prof. Norah Hoffmann.

Towards Accurate Mixed-Quantum Classical Simulations of Polaritonic Chemistry

A key challenge in studying polaritonic chemistry is accurately simulating the quantum dynamics of many molecules strongly coupled to confined light modes. While fully quantum mechanical calculations are possible for single molecules, they become computationally intractable when modeling the collective effects of multiple molecules - a crucial regime for understanding real experimental systems.

Mixed quantum-classical (MQC) methods offer a promising middle ground, treating the most important quantum effects while using classical approximations where appropriate to maintain computational feasibility. However, even for single-molecule systems, traditional MQC approaches have important limitations that need to be addressed.

MASH (Mapping Approach to Surface Hopping) combines the best of two traditional mixed quantum-classical methods. Traditional surface hopping evolves trajectories on single potential energy surfaces with stochastic hops between them, while Ehrenfest dynamics uses mean-field forces averaged over all surfaces. MASH maps quantum states to classical variables and evolves them continuously like Ehrenfest, but also includes surface hops to better capture quantum effects that are lost in pure mean-field evolution.

Our work makes two key advances:

1. We implement the Mapping Approach to Surface Hopping (MASH) - an improved MQC method developed by Dr. Jonathan Mannouch and Prof. Jeremy Richardson, refined further by Dr. Johan E. Runeson and Prof. David Manolopoulos that better captures quantum effects compared to conventional approaches

2. We incorporate a quantum treatment of the cavity mode itself, moving beyond the classical approximations typically used in previous studies

Comparison of reaction rates calculated using different simulation methods (Ehrenfest vs MASH) and cavity mode treatments (classical vs quantum). The vertical dashed line shows the resonant frequency. Error bars indicate 99% confidence intervals.

Our results show that combining MASH with a quantum cavity description provides the most accurate reaction rates in single-molecule test cases. However, we discovered that this approach can face challenges with “size-inconsistency” - where the dynamics at zero coupling depend on whether the cavity mode is quantized.

To address this, we developed ε-MASH, which prevents unphysical transitions between states with negligible coupling. This modification maintains accuracy while ensuring proper behavior in important limiting cases.

These methodological advances lay the groundwork for accurate and computationally tractable simulations of collective effects in polaritonic chemistry - a crucial step toward understanding and predicting how strong light-matter coupling can be harnessed to control chemical reactivity.

Paper can be found in Ref.  [1]

References

[1]

Hasyim, M. R., A. Mandal, and D. R. Reichman, Towards accurate mixed quantum classical simulations of vibrational polaritonic chemistry, (2025).