Here, SP3 is the number of sp 3 chain atoms (not including end carbons), SP2 is the number of sp 2 chain atoms (also not including end carbons), and RING is the number of fused-ring systems. The molecules in the liquid phase are more mobile than molecules in the solid phase, so the molecular flexibility should raise the entropy of the liquid phase more than that of the solid phase. This results in a higher entropy of melting, represented by a positive value for b.