Since many experiments are done in Fabry-Pérot cavities, Section VI discusses the Hamiltonians used for such systems. Since Fabry-Pérot cavities are made of flat mirrors, they have a quasi-continuous spectrum of photonic modes and typically hold many molecules, so it is helpful to use Hamiltonians that explicitly consider this. This section discusses the generalized multi-center dipole gauge Hamiltonian that considers many molecules as individual dipoles inside a cavity (Sect. VI A). Then, by making a series of approximations, the generalized Tavis-Cummings Hamiltonian is formulated, which can drastically reduce the computational cost of modeling these very complex systems (Sect. VI B).