Strain is present in a @M03986@ or @T06471@ if the energy is enhanced because of unfavourable bond lengths, bond angles or dihedral angles ('@E01886@') relative to a standard. It is quantitatively defined as the standard @E02141@ of a structure relative to a strainless structure (real or hypothetical) made up from the same atoms with the same types of bonding. (The @E02141@ of formation of cyclopropane is \(53.6\ \text{kJ mol}^{-1}\), whereas the @E02141@ of formation based on three 'normal' @M03881@ groups, from acyclic models, is \(-62\ \text{kJ mol}^{-1}\). On this basis cyclopropane is destabilized by ca. \(115\ \text{kJ mol}^{-1}\) of @ST07108@.)