文档介绍：2. Intermolecular Interactions
General Description
First, some general principles summarizing the general knowledge about in-
termolecular potentials will be presented. Chapter 3 gives a more detailed,
quantitative description of intermolecular interactions. Textbooks discussing
this subject include [126, 165, 185]. Later on I shall give an overview of the
work of other groups, related to semiempirical potentials.
The Hamiltonian of a plex is the sum of the ic and po-
tential energy of all electrons and nuclei. In the so-called Born–Oppenheimer
approximation the nuclear motion is separated from the electronic motion.
This is a reasonable assumption if the lighter electrons follow instantaneously
the motion of the nuclei. As a result any influence of the nuclear motion on
the electronic energy is separated and treated as an intramolecular potential
field V (r) in which the nuclei move [85]. If we want to discuss intermolecular
interactions we have to introduce the concept of “intermolecular potentials”.
The intermolecular potential is used to describe the binding between any pair
of molecules (A, B) or atoms. For the simple case of an atom–atom pair, the
binding energy is given by the difference between the energy of plex
and that of the two separated atoms:
VAB(R)=EAB (R) − EA − EB , ()
where R is the distance between atom A and atom B. However, if one or
both of the partners are molecules, the potential will depend on additional
coordinates describing the relative orientation of the molecules with respect
to one another. The simplest example of an atom–plex is shown
in Fig. . R, r and θ denote the distance between the atom and the center
of mass of the molecule, the intramolecular distance and the angle between
the intermolecular and intramolecular axes, respectively. The potential field
in which the nuclei move can now be described as V (r, R, θ). Similarly to
the Born–Oppenheimer approximation, we can separate the intermolecula