Pauli hardness of the van der Waals surface

How would an atom penetrate the van der Waals surface It depends on the particular molecule, surface point and atom. The helium atom seems to be a good probe, because of its simplicity and small size. The question may be more specific what is the value of the valence repulsion gradient or, alternatively, the interaction energy gradient, when the atomic probe penetrates perpendicularly at a given point of the van der Waals isosurface Such hardness depends on the particular spot on the isosurface and exhibits the symmetry of the molecule.  

The van der Waals surface might be modelled as one of the isosurfaces of the function D r) = exp(-/Ii -R/l) where the summation goes over the atoms of the molecule and the coefficients Ai and Bi depend not only on their kind (element), but also on their neighbourhood in the molecule. Therefore, we may propose 

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