This article introduces a new quantum chemistry method called Cavity Quantum Electrodynamics Complete Active Space Configuration Interaction (QED-CASCI) for modeling polariton chemistry. Polaritons are quasiparticles formed by the strong coupling of light and matter, and their study is important for understanding chemical reactions and properties. The QED-CASCI method combines ab initio quantum chemistry with cavity quantum electrodynamics to treat both electronic and photonic degrees of freedom on an equal footing. This approach allows for the calculation of ground- and excited-state polaritonic surfaces, providing a balanced description of strong correlation effects between electronic and photonic degrees of freedom. The method is based on the Pauli-Fierz Hamiltonian, which describes the interaction between a molecular system and a single photonic mode. The Hamiltonian is transformed into the coherent-state basis, which simplifies the calculation of the photonic part of the system. The QED-CASCI method is implemented in two forms: PN-QED-CASCI, which uses photon number states, and CS-QED-CASCI, which uses coherent states. These formulations allow for the efficient calculation of large active spaces, which are essential for accurately modeling the electronic structure of molecules. The article presents computational results for several model systems, including LiH, H2O2+, and BH3. These results demonstrate the effectiveness of the QED-CASCI method in capturing the behavior of polariton states and the impact of the photonic basis size on the accuracy of the calculations. The method is shown to provide accurate results for both ground- and excited-state potential energy surfaces, and it is capable of handling large active spaces with a high degree of accuracy. The results also highlight the importance of the photonic basis size in determining the accuracy of the calculations. For example, in the LiH system, increasing the size of the photonic basis leads to a significant reduction in the mean absolute error of the potential energy surfaces. Similarly, in the H2O2+ system, the energy error decreases as the size of the photonic basis increases, demonstrating the method's ability to handle charged systems with a high degree of accuracy. The article concludes that the QED-CASCI method is a promising approach for modeling polariton chemistry, providing a balanced description of strong correlation effects between electronic and photonic degrees of freedom. The method is capable of handling large active spaces and is particularly effective in capturing the behavior of polariton states. The results demonstrate the method's ability to accurately model the behavior of polariton chemistry, making it a valuable tool for future studies in this area.This article introduces a new quantum chemistry method called Cavity Quantum Electrodynamics Complete Active Space Configuration Interaction (QED-CASCI) for modeling polariton chemistry. Polaritons are quasiparticles formed by the strong coupling of light and matter, and their study is important for understanding chemical reactions and properties. The QED-CASCI method combines ab initio quantum chemistry with cavity quantum electrodynamics to treat both electronic and photonic degrees of freedom on an equal footing. This approach allows for the calculation of ground- and excited-state polaritonic surfaces, providing a balanced description of strong correlation effects between electronic and photonic degrees of freedom. The method is based on the Pauli-Fierz Hamiltonian, which describes the interaction between a molecular system and a single photonic mode. The Hamiltonian is transformed into the coherent-state basis, which simplifies the calculation of the photonic part of the system. The QED-CASCI method is implemented in two forms: PN-QED-CASCI, which uses photon number states, and CS-QED-CASCI, which uses coherent states. These formulations allow for the efficient calculation of large active spaces, which are essential for accurately modeling the electronic structure of molecules. The article presents computational results for several model systems, including LiH, H2O2+, and BH3. These results demonstrate the effectiveness of the QED-CASCI method in capturing the behavior of polariton states and the impact of the photonic basis size on the accuracy of the calculations. The method is shown to provide accurate results for both ground- and excited-state potential energy surfaces, and it is capable of handling large active spaces with a high degree of accuracy. The results also highlight the importance of the photonic basis size in determining the accuracy of the calculations. For example, in the LiH system, increasing the size of the photonic basis leads to a significant reduction in the mean absolute error of the potential energy surfaces. Similarly, in the H2O2+ system, the energy error decreases as the size of the photonic basis increases, demonstrating the method's ability to handle charged systems with a high degree of accuracy. The article concludes that the QED-CASCI method is a promising approach for modeling polariton chemistry, providing a balanced description of strong correlation effects between electronic and photonic degrees of freedom. The method is capable of handling large active spaces and is particularly effective in capturing the behavior of polariton states. The results demonstrate the method's ability to accurately model the behavior of polariton chemistry, making it a valuable tool for future studies in this area.