Coupled cluster theory for coupled electron-photon systems

Recent experiments show that properties of materials and energy landscapes of chemical reactions can change drastically when put inside a high-Q optical cavity. Understanding these phenomena require novel theoretical approaches, where both parts of the problem, light and matter, are treated on an equal quantum mechanical footing. Extension of existing electronic structure methods to include photons provides one route in this direction.

One prominent tool in electronic structure theory is coupled cluster (CC) theory which is often called the gold-standard in quantum chemistry, as it is a robust polynomial-scaling approach for handling weak correlations.

In the present work we develop methodology to extend CC theory to coupled electron-photon systems. We show benchmark results for model molecular hamiltonians coupled to cavity photons. By comparing to full configuration interaction results, we study the accuracy of the proposed method. We show that our method includes effects like multiphoton processes, which ab-initio methods so far fail to describe. Therefore, not only does our method go beyond standard quantum optical approaches, since it includes an accurate treatment of electronic structure, it also goes beyond existing methods such as QEDFT in terms of its description of light.