Page 200 - Strategies and Applications in Quantum Chemistry From Molecular Astrophysics to Molecular Engineer

HARTREE-FOCK MODEL TO THE LOWEST SINGLET AND TRIPLET EXCITED STATES 183

angle, was for many years an attractive problem for many scientists, in order to an-
alyze the spectrum structure of the transition which is allowed [16,17].
Next, and as an example of HPHF calculations, the potential energy curves for the
bending in the and states of methylene were determined into the HPHF ap-
proximation, using the Huzinaga-Dunning valence basis set with polarization orbitals
(d on the C atom, and p on the H atoms with 0.8 and 1.1 as exponent respectively),
and full geometry optimization. For this purpose, an orbital was substituted by
an one, into the fundamental configuration. The energy variations are given in
Table 1 for both states. The geometrical parameters encountered in this way are in
very good agreement with the experimental data. These are gathered in Table 2.

3.2. THE METHANAL MOLECULE
Methanal (formol) presents an additional degree of freedom: the out-of-plane wagging
of the oxygen atom. In its singlet ground, this molecule is planar. But, in its
triplet and singlet lowest exited and this molecule is pyramidal
with the possibility of inversion, because its electronic system is destabilized by
antibonding orbital. In order to test the performance of the HPHF model, to
determine the potential energy curves for the inversion seems to be illustrative.
HPHF calculations with full geometry optimization were performed for these lowest
excited states using the Huzinaga-Dunning valence basis set with polarization or-
bitals. The potential energy curves are given in Figure 1. As expected, a double
potential energy well is obtained for both excited states of methanal. In Table 3,
the geometrical parameters, as well as the inversion barrier, obtained with different
basis sets and different approaches are given for the first singlet state, It
is seen that the values for the barrier height are very basis dependent. Anyway, the
HPHF approach gives the best values into the same basis set, except for the SCF-CI
calculation which involves 17,000 configurations [18].
In Table 4, similar results are gathered for the first triplet excited state,
Here, the UHF model is seen to furnish slightly better results. The HPHF
model, however, yields a better value for