Level: 3rd
A chain contains a series of POINTs, which is usually atom positions, and is used to draw COILs and RIBBONs. See COIL, RIBBON, and POINT for detail about CHAIN
A chain must have at least four POINTs. Very short chains with less than four POINTs are simple ignored. This limitation comes from the Catmull-Rom method, which is used to intepolate POINTs.
Number of points inserted between adjacent two points with Catmull-Rom method.