The occupied eigenvectors are transformed into a localized  set of
M.O.s by a series of 2 by 2 rotations which maximize
is a direct measure of the number of centers involved
in the MO: thus, the value of
is 2.0 for H2, 3.0 for a
three-center bond and 1.0 for a lone pair. Higher degeneracies than
allowed by point group theory are readily obtained. For example, benzene
would give rise to a 6-fold degenerate C-H bond, a 6-fold degenerate C-C
sigma bond and a three-fold degenerate C-C pi bond. In principle, there
is no single step method to unambiguously obtain the most localized set
of M.O.s in systems where several canonical structures are possible,
just as no simple method exists for finding the most stable conformer of
some large compound. However, the localized bonds generated will
normally be quite acceptable for routine applications.
Localized orbitals are also known as Natural Bond Orbitals or NBO.