Note: Due to variations in atom naming conventions, in rare instances VMD may improperly assign VDW radii to specific atoms, since VMD determines each atom type based on the first letter forming its name. For example, VMD would assume an atom named ``HG'' to be a hydrogen rather than a mercury. If this happens, you are always free to redefine the radii, using a syntax much like that below:
set sel [atomselect top ``name HG''] $sel set radius 1.9