Chemistry Reference
In-Depth Information
11
Atomic and Molecular
Interactions
In this chapter we shall present first (Section 11.1) the elementary RS
perturbation theory for the interaction between two ground-state H
atoms, taking the interatomic potential V as a first-order perturbation.
The interacting atoms are assumed to be sufficiently far apart so that, in
the first approximation, we can ignore the effect of exchanging identical
electrons between different atoms, as required by Pauli's antisymmetry
principle. As a consequence, the two-electronwavefunctions will be taken
in the form of simple orbital products for the two electrons, insisting on
the fact that electron 1 belongs to atomA and electron 2 to atom B, giving
what is called theCoulombic interatomic energy, whichwill be considered
up to second order in V. The possibility of including electron exchange in
the first-order theory is examined in some detail elsewhere (Magnasco,
2007), where it is seen that it is equivalent to the HL theory of Chapter 9.
The expansion of the interatomic potential V into inverse powers of the
internuclear distance R, giving the so-called multipole expansion of the
potential, is then examined in Section 11.2, with particular emphasis on
the calculation of the leading term of the London attraction between
two H atoms. These considerations are then extended to molecules
(Section 11.3), while a short discussion on the nature of the VdW and
hydrogen bonds concludes the chapter.
