Usually for acyclic systems trans isomers are more stable than cis isomers. This difference is attributed to the unfavorable steric interaction of the substituents in the cis isomer. Therefore, trans isomers have a less-exothermic heat of combustion, indicating higher thermochemical stability. In the Benson heat of formation group additivity dataset, cis isomers suffer a 1.10 kcal/mol stability penalty. Exceptions to this rule exist, such as 1,2-difluoroethylene, 1,2-difluorodiazene (FN=NF), and several other halogen- and oxygen-substituted ethylenes. In these cases, the cis isomer is more stable than the trans isomer. This phenomenon is called the cis effect.