A perlurbnlion
theory for intcrmolccular interactions is dcvcIc+! with wave functions of arbitrary symmetry. Firsl-nnd second-order expressions are lvritten out. For the sake of simplicity in practical calculntions.
Lhc Unsiild npproximntion is introduced in the second-order cncrgy.
i. INTRODUCTION
Ikermolecular and interatomic 1:lteraction.s are usually distinguiskd in two types: At small distances the "chemical" interactions are importa!?!. They are calculated i? first-order perturbation theory, taking the Pauli exclusion principle into account by using properl, antisymmetrized wave functions (e. g. Heitler-London).
At distances large compared to the molecular, atomic diameters these inicractions are negligibly small compared to the, "physicai', Van der Waals interactions. The latter are calculated in second-order perturbation theory with wave functions that are simple products of molecular, atomic wave functions.
Since both types of interactions are of quantum theoretical nature this distinction has LO be justified an& it will certainly not be valid for intermedist'e @stances.
The firs! to develop a perturbation theory for inttzratomic interactions with antisymmetric wave fLr;ctions were Eiserzchitz and London [l]. After them scch a perturbation theory or-a variational procedure that is quite similar to it was used by Margenau [2], Moffitt [3], McGinnies and Jansen [a], Dalgarno and Lynn [5], Hirschfelder and Silbey [G]. We thizk that the treatment of Eisenschitz an:! London is so complete and mathematically elegant that it deserves to be written out in a'more modern, somewhat generalized language.
There are two basic diffevences that distkguish this perturbation theory :vith antisymmetric wave functions from the usual per!crbntion theory. The unperturbed Hamiltonian, If,, is a sum of * On leave of absence from 'Jnilever Research Lnboratories, Vlaardingen,