3. Choice of atomic orbitals or basis functions

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...