Definitions of (a) the van der Waals surface and (b) the molecular surface. In this two-dimensional illustration, atoms are represented by gray disks. In (a), the exposed boundaries of the disks, shown in dark, constitute the van der Waals surface. In (b), a spherical solvent probe is rolled around the solute molecule. In addition to the van der Waals spheres, small crevices inaccessible to the solvent probe are now part of the solute region. The boundary of this filled-up solute region, shown in dark, is the molecular surface.