double OEGetPaulingVdWRadius(unsigned int elemno)
Return the Van der Waals radius for a particular atomic number as
tabulated in Linus Pauling, ``The Nature of the Chemical Bond'', 3rd
Edition, 1960. There are several experimental values for Van der Waals
radii (see also OEGetBondiVdWRadii) and many additional values
used in molecular mechanics forcefields (including united-atom VdW radii),
so there is often no definitive notion of Van der Waals radius.