Page 294 - Strategies and Applications in Quantum Chemistry From Molecular Astrophysics to Molecular Engineer

AB INITIO CALCULATIONS OF POLARIZABILITIES IN MOLECULES 277

factor makes an extrapolation procedure possible in critical cases (for example when
the low–lying states are of Rydberg character as the states of CO).
This calculation has shown the importance of the basis set and in particular the
polarization functions necessary in such computations. We have studied this problem
through the calculation of the static polarizability and even hyperpolarizability. The
very good results of the hyperpolarizabilities obtained for various systems give proof
of the ability of our approach based on suitable polarization functions derived from
an hydrogenic model. Field–induced polarization functions have been constructed
from the first– and second–order perturbed hydrogenic wavefunctions in which the
exponent is determined by optimization with the maximum polarizability criterion.
We have demonstrated the necessity of describing the wavefunction the best we can, so
that the polarization functions participate solely in the calculation of polarizabilities
or hyperpolarizabilities.

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