One question always arises that how many basis functions should be used to describe the atomic system. The basis set selection process requires a lot of expertise. In practice, we should start with only contracted functions that are enough to describe the atomic system, which is known as a minimum basis set. The minimum basis functions which are needed to describe the individual atom is called the Minimal Basis set (MBS). For example, hydrogen only needs a single s-function while first row elements of the periodic table require two sfunction and a set of p-functions (2px, 2py and 2pz) to describe its occupied atomic orbitals.