  Advances in nanophotonics, quantum optics, and low-dimensional materials have
enabled precise control of light-matter interactions down to the nanoscale.
Combining concepts from each of these fields, there is now an opportunity to
create and manipulate photonic matter via strong coupling of molecules to the
electromagnetic field. Towards this goal, here we introduce a first principles
framework to calculate polaritonic excited-state potential-energy surfaces for
strongly coupled light-matter systems. In particular, we demonstrate the
applicability of our methodology by calculating the polaritonic excited-state
manifold of a Formaldehyde molecule strongly coupled to an optical cavity. This
proof-of-concept calculation shows how strong coupling can be exploited to
alter photochemical reaction pathways by influencing avoided crossings.
Therefore, by introducing an ab initio method to calculate excited-state
potential-energy surfaces, our work opens a new avenue for the field of
polaritonic chemistry.
