Electron densities are shown for the mirror plane of the global IAA
minimum, with color changes at
0.01, 0.02, ..., 0.10, 0.15, 0.20, ..., 0.60, 0.85, 1.10, ...,
3.10, and 5.00 electrons/Bohr³.
Saddle points in the electron density identify chemical bonds ( "What
is a bond?", R. F. W. Bader);
the density values in these points are a measure for the strength
of the corresponding bonds. Basis set differences become obvious in
the six-membered phenyl ring of the indole nucleus:
in the 3-21G and 4-31G description, stronger and weaker C-C bonds alternate,
resembling the alternating single- and double-bond picture,
whereas in the 6-31G* and the
6-311G** cases all six aromatic bonds are of equal strength.