Page 187 - Strategies and Applications in Quantum Chemistry From Molecular Astrophysics to Molecular Engineer

170 C. AMOVILLI AND R. McWEENY

the core ionization. The absolute energy values are of course somewhat inferior to
those obtained with a basis (cf. the results for in Table 1) but the ionization
energies, shown in Table 4, are still in very satisfactory agreement with experiment.
Even the chemical shifts for fluoro-substitution are very close to those observed: what
is more surprising is that the shifts predicted by the Koopmans theorem are also quite
satisfactory, even though the ionization energies are in error by 10-15 eV. Whether
the Koopmans theorem will retain its apparent predictive value in situations where
the chemical shift is much smaller remains an open question.

4. Conclusions and further applications
It is evident that the method of calculation used in this work provides an extremely
simple approach to the interpretation of ESCA results for ionization from an atomic
K shell, in spite of the fact that the state of the ion plus the ejected electron lies
high in the energy continuum of the neutral molecule. More sophisticated methods of
dealing with such states are of course available (see Agren et al [7]) but, whilst capable
of giving excellent results for valence electron ionizations (including also intensities
and vibrational fine structure), encounter considerable difficulty in treating core-hole
states, where relaxation effects are very severe. The simple model used here, on
the other hand, is particularly well adapted to the study of these ‘deep’ ionizations
and gives an immediate and transparent interpretation of the relaxation effects in
terms of scaling (contraction) of the valence orbitals. It is also possible to extend
the approach in various ways; for example, for open-shell molecules, the states of the