Page 67 - Mechanism and Theory in Organic Chemistry

The extended Huckel method, developed by Hoffmann," has the following
characteristics :
1. All valence orbitals are included in the basis. The method is not restricted
to .rr systems.
2. All overlaps S,j are calculated and included.
3. Energies H,, of an electron in each of the basis orbitals are estimated
empirically from spectroscopic data.
4. Interactions Hij are approximated as a simple function of St,, Hi,, and
Hjj.
This method provides a practical way of carrying out rapid calculations on
moderately large organic molecules. Although energies are still not particularly
reliable, energy differences within a series of similar compounds are revealing.
Huckel and extended Huckel methods are termed semi-empirical because
they rely on experimental data for the quantification of parameters. There are
other semi-empirical methods, such as CNDO, MINDO, INDO, in which experi-
mental data are still used, but more care is taken in evaluating the Hi,. These
methods are self-consistent field procedures based on X,,,. They are discussed
in various works on molecular orbital theory.'

AB INITIO METHODS
Ab initio methods calculate all quantities needed from first principles and use no
experimentally determined parameters. The computations require more machine
time and are therefore more expensive than semi-empirical ones. Good
energies can now be obtained for small molecules by these techniques. The reader
is referred to specialized treatments for further informati0n.j

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R. Hoffmann, J. Chem. Phys., 39, 1397 (1963).
'See note 2, p. 10.
For an introduction, see W. G. Richards and J. A. Horsley, Ab initio Molecular Orbital Calculations
for Chemists, Oxford University Press, London, 1970.