Page 281 - Strategies and Applications in Quantum Chemistry From Molecular Astrophysics to Molecular Engineer

264 M. TADJEDDINE AND J. P. FLAMENT
For a heteronuclear diatomique molecule of symmetry (z being the molecular
axis), it becomes :

with :

where is the electron number. As tends to zero, the constants tend to

The normalization condition imposes to move the origin to the center of
electronic charge thus, the polarizability may be written very
simply in the limit of zero frequency :

2.1.3. A mixed approach

The idea to combine a method only polynomial (Eq.6 with and ) with
the SCF–CI procedure (Eq.5 with ) has been initially developed for
the calculation of magnetic observables (9) and later for the electric ones (10). Thus,
the first–order perturbed wavefunction is given by :

and the component of the polarizability tensor becomes :

The calculations of the and constants lead to a system of linear equations
similar to that of the SCF–CI method, but with three more lines and columns corre-
sponding to the coupling of the polynomial function with the electric field perturba-
tion. The methodology and computational details have already been discussed (1);
we stress two points : the role of the dipolar factor, the nature and the number of
the excited states to include in the summation.