The Born-Oppenheimer approximation treats the nuclei as classical and stationary particles, and the electrons are said to be on the Born-Oppenheimer surface. Under this approximation, the nuclear kinetic energy is neglected and repulsion between the nuclei is considered as constant for fixed nuclei. The molecular Hamiltonian (given in Equation ) is then reduced to the electronic Hamiltonian (Ĥ 𝒆𝒍𝒆𝒄 ) which contains only electronic part given as;