Olefin metathesis involves little change in enthalpy for unstrained alkenes. Some elastomers which are amorphous in the unstrained state undergo rapid crystallization upon stretching. In BCC metals, interstitial sites of an unstrained lattice are equally favorable. Let a be the lattice parameter of the unstrained solid of the average composition c. The value 658.6 kJ per mole is obtained from an unstrained long-chain alkane. Unstrained "Skolithos" structures are normal to the plane of the bed.