There are three main types of orbitals. One of the basis sets is hydrogen-like orbitals, which are derived for a one-electron atom, so they are unsuitable for many-electron atoms. It yields the absolute explications for the hydrogen atom. It cannot be used for a complex system even if it is mutually orthogonal as an advantage.65 As for the Slater-type orbitals (STOs), several STOs are combined to symbolise each HF orbital . It has also been completed and set to be used for several calculations. Yet, it is not reciprocally orthogonal and a time saver because of an increasing number of functions, and there is no node as a weak point. Therefore, nodes can be determined by combining the STOs as a straight line. Besides, it highly reflects the orbitals for the hydrogen . It is better to use STOs in semi-empirical methods and calculation essential high accuracy . STOs are more accurate, yet they need a longer time. It can be solved by combining GTOs linearly, which may mimic the STOs. These kinds of basis sets can be called "STO-nG". It is also possible to select STOs for density functional methods. The STO-3G basis is the most commonly used minimum basis set . The basis set should not be used because of its size. Therefore, the bigger the basis is usually better than the smaller. The other standard basis sets of this type are STO-4G and STO-6G.