A linear FE model with electron—electron and electron—core interaction appeared to be fairly accurate in reproducing the experimental data for (i) lowest excited singlet—triplet separation in polyenes, (ii) twisting frequencies of ethylene, and (iii) location of the absorption maxima in cumulenes. For calculations of (i) and (ii), electron—electron interaction has been assumed to play the dominant role. With the same assumption made, cis—trans isomerization of polyenes was found to be easier with long chains than with short. It also is easier about a central double bond in the chain than about an outer double bond. In the case of a single double bond, an important part of the energy path in isomerization is that for the triplet state; however, the role of triplet states in isomerization should diminish rapidly with an increase in the number of double bonds in the chain. With ethylene as the example, the effect of pi‐electron correlations in the FE model was briefly investigated, and the correlation for th...