Page 286 - Strategies and Applications in Quantum Chemistry From Molecular Astrophysics to Molecular Engineer

AB INITIO CALCULATIONS OF POLARIZABILITIES IN MOLECULES 269
where and are the well known Dunham constants and and the values
of the first and second derivatives of the polarizability calculated at the equilibrium
geometry.
By including the effect of the rotation (at a given T temperature, for the level v = 0)
in a perturbation calculation, we have obtained (5) :

where we have introduced the dimensionless constant c like :

which characterizes the molecule (m is its reduced mass).
Moreover, for the observables depending on external electric field, its specific effect
has to be investigated : the electric field induces new terms in the nuclear Hamilto-
nian, due to the change of equilibrium geometry and the nuclear motion perturbation.
Pandey and Santry (14) has brought to the fore this effect and calculated the correc-
tion which only concerns the parallel component. It is represented by the following
expression :

where is the value of the first derivative of the dipole moment calculated at the
equilibrium geometry. On table 2 we have reported the results obtained for the

static polarizability of CO : the correction for the perpendicular component may be
neglected; for the parallel component, the vibronic correction mainly originates from
the effect of the electric field cannot be neglected at all.

2.6. RESULTS FOR THE POLARIZABILITY OF CO
The quality of electronic calculations is confirmed by the very good agreement of the
resonance energies for both components if we compared to the experimental ones,
as shown on table 3.