 The surface is constructed by tracing the center of an imaginary ball, its radius that of a water molecule (taken as 1.4 Å), as it rolls over the van der Waals surfaces of the proteins.Thus defined, the surface is continuous and each point on it is unambiguously associated with a specific protein atom (the nearest).The Lee & Richards definition has been widely adopted as the standard measure for solvent accessibility, for instance to evaluate exposure per residue as a percentage of accessible vs total surface area, and as the basis of the buried-surface-area method for estimating the energetics of protein/protein contacts.