These methods provide approximate, but accurate, solutions of the Schrödinger equation whose analytical solution cannot be found for multi-electron molecules. Since the coupled dynamics of the nuclei and electrons is a complex problem, the Born-Oppenheimer approximation is invoked to decouple the electrons and nuclei. The electronic structure is then calculated keeping the nuclei stationary. The standard procedure employed in these methods is approximation of the Hamil-tonian to account for all the important contributions to the electronic energy. The following two subsections describe the methods in detail.