Page 287 - Strategies and Applications in Quantum Chemistry From Molecular Astrophysics to Molecular Engineer

270 M. TADJEDDINE AND J. P. FLAMENT

Moreover, the values obtained for the dynamic polarizability by varying the wave-
length are in good agreement with experiment (1). Table 4 resumes
the results obtained for the static polarizability of CO :

1. On the first line, we have reported our results (1) obtained with the spectrocopic
states, the dipolar factor and the extrapolation procedure. In order
to compare them with the experimental results (last line) we have corrected
them by taking into account the vibronic coupling –temperature and electric
field dependence– as developed before (second line). The parallel component,
is now in excellent agreement with experiment.
2. The two following lines present the results obtained later by Rérat et al. (17) :
the method consists in adding one more term in the expression of given by
Eq.14. He keeps the dipolar factor; from the summation on the spectroscopic
states he retains only the first one ofthe symmetryof interest, thus there is
no extrapolation procedure; on the other hand, he adds the Slater determinants
which contribute to the perturbation of the ground state by the operators
and he takes into account the non–orthogonality of the zeroth and
first–order perturbed wavefunctions. Their results show an improvement for
both a components, in particular for anisotropy.

3. These results are compared with those obtained by Oddershede and Svendsen
(18) using SOPPA or Sunil and Jordan (19) using MP4 or a coupled cluster
approach, but without vibronic correction.

3.Determination of the polarization functions

In order to overcome the optimization process of the (hyper) polarizabilities calcula-
tions, we have been led to deeply study the perturbational and variational methods
and in particular the variation–perturbation treatment introduced by Hylleras (20)
since 1930. We will not develop here the theoretical framework of the recent study
of N. El Bakali Kassimi (21). We propose criteria for generating adequate sets of
polarization functions necessary to calculate (hyper) polarizabilities.
As our computations use the HONDO/8 program (22) which is based on the CPHF
(Coupled Perturbed Hartree Fock) method (23) we begin by briefly recalling this
method.