SASA is calculated via the "rolling ball" algorithm of Shrake & Rupey, in which a solvent "probe" sphere is rolled along the outer surface of a union of spheres centered at each atom. The area of the manifold traced by the center of this probe is then approximated by a mesh of sample points on the probes surface. Both the probe sphere and the atom sphere are constructed to have radii equal to the extent of their respective van der Waals interactions (so-called "van der Waals radii"); for an aqueous solvent for example, this yields