Coupled Cluster theory is a sophisticated method used in quantum chemistry to describe the electronic structure of molecules. It improves upon simpler models by considering the interactions between electrons more comprehensively. This approach involves creating a mathematical framework that accounts for the correlation between electron pairs, allowing for more accurate predictions of molecular properties. The core idea of Coupled Cluster theory is to express the wave function of a system as a combination of reference states, typically the Hartree-Fock state, and additional excitations. This enables chemists to capture the effects of electron correlation, making it a powerful tool for studying complex systems in fields like materials science and drug design.