Single-, Multiple-, and Split-Valence

The STO-3G basis set is what is known as a single- basis set, or, more commonly, a minimal basis set. This nomenclature implies that there is one and only one basis function 

and Sp are added to the second-row set, making a total of nine functions, etc. This number is the absolute minimum required, and it is certainly nowhere near the infinite basis set limit. Other minimal basis sets include the MINI sets of Huzinaga and co-workers, which are named MINI-1, MINI-2, etc., and vary in tlie number of primitives used for different kinds of functions. 

Valence orbitals, on the other hand, can vary widely as a function of chemical bonding. Atoms bonded to significantly more electronegative elements take on partial positive charge 

The Pople basis sets have seen sufficient use in the literature that certain trends have clearly emerged. While a more complete discussion of the utility of HF theory and its basis-set dependence appears at the end of this chapter, we note here that, in general, the 4-3IG basis set is inferior to the less expensive 3-21G, so there is little point in ever using it. The 6-21G basis set is obsolete. 

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