Some Properties of Van der Waals Molecules

5 Some Properties of Van der Waals Molecules 5.1 Orimtational Dqiendence 

In the very same way as the Bom-Oppenheimer approximation allows the definition of a potential energy surface for a Van der Waals molecule, it enables, too, the conceit of an interaction tensor field. This is a field dependent on the relative coordinates of the monomers and transforming as a tensor under rotation of the complex as a ole. (The potential energy surface is an example of a rank zero interaction tensor field). In the case of tensor fields it is also convenient to base the theory on irreducible tensors and to use an e7q ansion in terms of a complete set of functions of the five angular coordinates describing a Van der Waals dimer. 

The generalization of the scalar-valued angular functions (1 b) to arbitrary rank J is  

It is possible to apply the mxiltipole e q ansion and perturbation theory in order to derive long range expressions for Tj (R), thus relating this quantity to monomer I operties. A simple example of sudi a procedure can be found in the appendix of ref. where the induction contribution to the dipole moment (J = 1) of an arbitrary Van der Waals dimer has been evaluated. 

Because not much is known experimentally about general interaction tensors, and especially not about their long range behaviour, we will not pursue this line of approach, but rather give a brief review of the existing work which has concentrated on two different tensors the pair dipole (order 1 tensor) and the pair polarizability (order 2 plus order 0 tensor). 

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