Page 292 - Strategies and Applications in Quantum Chemistry From Molecular Astrophysics to Molecular Engineer

AB INITIO CALCULATIONS OF POLARIZABILITIES IN MOLECULES 275

This last result which will be verified with the following applications is a consequence
of our choice for the polarization functions. In effect, the STOs have nodeless radial
part and they all combine in phase in so that the resulting polarization function
is also nodeless and can be approximately modeled by only one or two STOs with
suitable exponents.

3.3.2. Dihydrogen

If the values published for converge quite well (6.45 for and 4.5–4.6 for in
Ref. 38–40), nothing similar appears for components : 330 (39) 687 (38) for
such discrepancies
exist, though there are actually p and d orbitals, required for and calculations,
in all the basis sets used. This evidences the extreme sensibility of to the quality
of the wavefunction.
Mulliken (41) distinguishes two kinds of polarization. He calls ”Coulomb polar-
ization” what we are concerned with in this paper : the polarization produced by
an electric field, and he calls ”valence polarization” : a kind of polarization du to
quantum–mechanical valence forces. In order to correctly describe the chemical bond
in it is necessary to include the ”valence polarization” function as soon as one
calculates energy with the unperturbed function (i.e. the 2p orbital).

For this calculation we used the basis set ls,2s,2p of Fraga and Ransil (35) which
gives near HF limit quality for energy The polarization
functions were derived from the 1s orbital only, like in He calculations. Their expo-
nent was optimized using the maximum probability criterion Table 7
presents the obtained results.
Now with the 2p valence polarization, it is possible to partly describe the polarizabil-
ity since the first step of calculation with the unperturbed wavefunction, especially
the parallel component which is generally easier to calculate in CPHF. The optimized
values of are excellent at the second step with