Finding the solution of Schrödinger equation for simple molecular species such as H2 + is quite easy; however, dealing with the molecular systems is very complicated. Born-Oppenheimer approximation plays an important role in the case of electronic structure calculation of the molecular/cluster systems due to simplification of the molecular Schrödinger equation . The mass of a proton is 1836 times higher than an electron, due to which the nuclei will have a much smaller velocity compared to the electron. On the time scale of the nuclear motion, we can consider that the electrons will relax to the ground state which can find out by using equation 2. This approximation makes it possible to separate the motion of the atomic nuclei and the motion of the electrons within the molecule. The Born-Oppenheimer approximation is primarily based on the following assumptions;