As we know the outer shell electrons, i.e. valance electrons, usually participate in the chemical bond formation, therefore it is necessary to distinguish the core and valence electron considering the basis functions allocation. More the sets of the basis function (larger zeta) allocated for the valance electrons in comparison to the core electrons, the higher will be the accuracy of the results. Basis sets are categorised, e.g. valence double, triple, quadruple-zeta, on basis of the available number of basis functions for defining the valance atomic orbital.