[Chimera-users] Molecular Representations

Dear all, I have one question about the Molecular Representations. In chimera, molecule can be represented by ball and sphere. Sphere is with the VDW radius for atoms and stick bonds. What about the ball? What is the ball size? I tried to measure the distance between two molecules. I used command line of "measure distance #1 #2". It shows closest distance between two sets of atoms. When I made surface map for the same two molecules, I used the same command, it showed different number. I understand it is the distance of VDW. But what is the distance between atoms if I used PDB file instead of surface map? Which one is more reliable? Thanks, Jian