`VDW' draws the atoms as spheres. The radius used is the van der Waals radius multiplied by a user-selectable scaling factor. The sphere resolution determines how finely to tessellate the spheres that are drawn. Drawing spheres takes some time, since they are built from many polygons.
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