Møller–Plesset Perturbation Theory is a mathematical approach used in quantum chemistry to improve the accuracy of calculations for the energy levels of many-electron systems. It builds on the Hartree-Fock method by treating electron-electron interactions as small perturbations, allowing for a systematic way to include these effects in energy calculations. The theory is typically expressed in a series expansion, where the first-order term corresponds to the Hartree-Fock energy, and higher-order terms account for additional interactions. This method helps chemists and physicists better understand molecular properties and behaviors by providing more precise energy estimates.