sentences:  In 1952, Robert Corey and Linus Pauling described accurate scale models of molecules which they had built at Caltech.In their models, they envisioned the surface of the molecule as being determined by the van der Waals radius of each atom of the molecule, and crafted atoms as hardwood spheres of diameter proportional to each atom's van der Waals radius, in the scale 1 inch = 1 Å.To allow bonds between atoms a portion of each sphere was cut away to create a pair of matching flat faces, with the cuts dimensioned so that the distance between sphere centers was proportional to the lengths of standard types of chemical bonds.