This lecture covers the basis sets errors in computational chemistry, including the truncation error in Hartree-Fock energy, basis set superposition error, and counterpoise correction. It explains the differences between restricted and unrestricted Hartree-Fock methods, as well as the importance of post-Hartree-Fock methods in treating electron correlation. The lecture also delves into configuration interaction, coupled cluster methods, and perturbation theory for electron correlation. Multiconfigurational methods and composite approaches for approximating highly accurate results are discussed, along with the pros and cons of various ab initio methods.