to accommodate all their electrons. For the earlier part of the periodic table
(hydrogen to argon) each atom has one basis function corresponding to its usual
atomic orbital description, with the proviso that the orbitals used by the later atoms
of a row are available to all those of the row. A hydrogen or helium atom has a 1s
basis function. Each “first-row” atom (lithium to neon) has a 1s,a2s, and a 2px,2py
and 2pzfunction, giving five basis functions for each of these atoms: although
lithium and beryllium are often not thought of as usingporbitals, all the atoms of
this row are given the same basis, because this has been found to work better than a
literally minimum basis set. Second-row atoms (sodium to argon) have a 1sand a
2 s, as well as three 2pfunctions, plus a3sand three 3pfunctions, giving nine basis
functions. In the third row, potassium and calcium, as expected, have the nine
functions of the previous row, plus a 4sand three 4pfunctions, for a total of 13 basis
b, 3-21GFig. 5.13(continued)
240 5 Ab initio Calculations