Résumé : Total molecular energies (Etot), zero point energies (EZPE), thermal corrections (Eth) to 298 K, and relative energies (Erel) are given for a data set of more than 200 conformers/rotamers/transition states of the unsubstituted linear polyenes, C2 nH2 n +2(n = 2–14), calculated at the B3LYP/6-311++G(d,p) level. For some shorter polyenes additional MP2(FC)/aug-cc-pVDZ and MP2(FC)/aug-cc-pVTZ optimizations, and single point MP4(SDQ)(FC)/aug-cc-pVTZ//MP2(FC)/aug-cc-pVTZ and MP4(SDTQ)(FC)/aug-cc-pVQZ//MP2(FC)/aug-cc-pVTZ calculations were performed. At all levels, Etot is lowest for the planar all-trans conformers. Planar mono-Cis (about a C[dbnd]C bond) conformers lie in a narrow energy range above the corresponding all-trans conformers at both the B3LYP/6–311++G(d,p) (∼1.7–2.1 kcal/mol) and the MP2(FC)/aug-cc-pVDZ (∼1.3 kcal/mol) levels. Di-Cis conformers are ∼1.3–1.5 kcal/mol higher in energy than mono-Cis conformers at the MP2(FC)/aug-cc-pVDZ level and mono-gauche (about a C–C bond) rotamers are ∼0.5 kcal/mol higher in energy than di-Cis conformers. The energetic order of the di-Cis and mono-gauche forms is reversed at the B3LYP/6–311++G(d,p) level.