Choice of atomic orbitals or basis functions
Molecular modeling - Theoretical basis (part 2)

In practice, the choice of the nature and number of atomic orbitals whose linear combinations can be used to construct molecular orbitals is very important: these basic functions, centered on the various nuclei, must provide the "flexibility" that is essential if the SCF procedure is to lead to a wave function representing a ground state that is sufficiently close to the Hartree-Fock limit, for example, at the cost of a reasonable computational effort. A certain a priori empirical knowledge of the chemical properties of the systems modeled, concerning, for example, the polarizability of atoms and the polarity of bonds, is a valuable guide in these choices. Practitioners have accumulated considerable know-how in this respect, which is generally incorporated into the major ab initio calculation software packages, some of which, such as Gaussian, have rapidly established themselves as "de...