The excess energy due to @S06002@ of a molecular entity or @T06468@ structure, i.e. distortions relative to a reference (real or hypothetical) 'strainless' structure with the standard bond lengths, bond angles and dihedral angles. The @S06037@ energy components involve the following destabilizing terms: non-bonded repulsions, [email protected]@ distortions, bond stretch or compression, rotation around or twisting of double bonds, and electrostatic @S06037@. In general, the contributions of these components are inseparable and interdependent. A quantitative assessment of @S06037@ and @S06037@ energies can be made by taking the difference between the heat of formation of the substance under consideration and that of a hypothetical @[email protected] model. Several approaches to the assessment of @S06037@ energies have been developed based on the use of energies of isodesmic and homodesmotic reactions and on the so-called 'strainless increments', i.e. heats of formation of certain groups (CH3, CH2, CH, C etc).