What is an appropriate form for an -electron wavefunction? The simplest
solution would be a product of one-particle functions (``orbitals''):

(28) |

Unfortunately, the Hartree product is not a suitable wavefunction because it ignores the antisymmetry principle (quantum mechanics postulate #6). Since electrons are fermions, the electronic wavefunction must be antisymmetric with respect to the interchange of coordinates of any pair of electrons. This is not the case for the Hartree Product.

If we simplify for a moment to the case of two electrons,
we can see how to make the wavefunction antisymmetric:

(29) |

(30) |

Note a nice feature of this; if we try to put two electrons in the same orbital at the same time (i.e., set ), then . This is just a more sophisticated statement of the

This strategy can be generalized to electrons using determinants.

Since we can always construct a determinant (within a sign)
if we just know the list of the occupied orbitals
, we can write it in
shorthand in a ket symbol as
or even more simply as
. Note that we have
dropped the normalization factor. It's still there, but now it's
just *implied*!

How do we get the orbitals which make up the Slater determinant? This is the role of Hartree-Fock theory, which shows how to use the Variational Theorem to use those orbitals which minimize the total electronic energy. Typically, the spatial orbitals are expanded as a linear combination of contracted Gaussian-type functions centered on the various atoms (the linear combination of atomic orbitals molecular orbital or LCAO method). This allows one to transform the integro-differential equations of Hartree-Fock theory into linear algebra equations by the so-called Hartree-Fock-Roothan procedure.

How could the wavefunction be made more flexible? There are two ways: (1)
use a larger atomic orbital basis set, so that even better molecular
orbitals can be obtained; (2) write the wavefunction as a linear combination
of different Slater determinants with different orbitals. The latter
approach is used in the post-Hartree-Fock *electron correlation
methods* such as configuration interaction, many-body perturbation theory,
and the coupled-cluster method.