• ab initio QED
• light-matter strong coupling
• polaritonic chemistry
• polaritonic properties
• polaritonic response theory
• TD-QED-HF
• TD-SC-QED-HF

In the last decade, there has been an increasing interest among the chemistry and physics community toward polaritonic devices, which via a strong interaction between photons and molecules provide a non-invasive way to engineer chemical processes.
In this thesis, a response theory for hybrid light-matter states has been developed, providing a fully consistent QED tool for the computation of linear response properties and polaritonic potential energy surfaces.
At first, an exact formal framework for the computation of polaritonic properties is derived and thoroughly discussed. Particular care is devoted to the problem of the orientational molecular disorder in quantum cavities and to the definition of response functions which describe the crosstalk between molecular and photonic degrees of freedom.
To model realistic chemical systems, the approximate linear response theory for two ab initio polaritonic methods, QED-HF and Strong Coupling (SC)-QED-HF, has been developed. The developed time-dependent parametrizations parallel the response equations routinely employed in quantum chemistry. The linear response equations have been implemented in a development version of the eT program. The models have been validated by studying para-nitroaniline, formaldehyde and ethylene as test systems. Computing the absorption spectra for these systems, the effects of the molecular orientation inside the cavity and collective effects increasing the number of molecules have been investigated.