Potential surfaces of Van der Waals molecules N2 2 and

We have studied the potential hypersurfaces for the Van der Waals dimers (Nj) and by varying all the independent internal coordinates (in the rigid molecule approximation, 3 angles for (Nj), 5 angles for (CjH ) and the distance R in both cases). It is of course not possible to present the complete surfaces pictorially we have displayed in figs. 9 and 10 some typical cuts through the surface of (N2)2. 

Expecially fig. 10 contains much information since the distance was varied to find the energy minimum for each orientation (0, 0, ). In the figs. 4 and 5 and 

Leaving aside this dynamical problem, we can make some further remarks about the equilibrium structure of Van der Waals molecules. Some attempts have been made to predict this structure from the molecular properties, multipole moments, polarizabilities, which are reflected in the long range interactions (electrostatic, dispersion). Other authors have assumed that the equilibrium structure of Van der Waals dimers resembles the structure of nearest neighbour pairs in molecular crystals. The latter approach could possibly be justified by packing considerations (short range repulsion). An example of the first approach is the prediction of a T-shaped (0 = 90°, 0 = j)A = 0°) equilibrium structure for the N2-dimer, mainly 

Experimental crystal structures, see ref. , AE calculated with the atom—atom potential  

In fig. 5 we see that, indeed, the T-shaped and the staggered parallel structure have maximum electrostatic attraction. The dispersion energy is most favourable, of course, for the linear structure. For distances in the neighbourhood of the (isotropic) Van der Waals minimum the (short range) exchange repulsion is the dominant anisotropic term, however. Since it increases very steeply when the molecular charge clouds start to overlap (especially in the linear structure 0 = 0g = 4 a — 1 

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