A value of −0.33 eV or −7.6 kcal mol−1 has been obtained for the vertical delocalisation energy of trans-1,3-butadiene from a nonempirical molecular orbital calculation on the π system. The result agrees well enough with ab initio calculations to suggest that a simplified approach need not be semiempirical. In a basis of orthogonalised atomic orbitals the central bond order is found to be 0.295 (Huckel value 0.447) for the delocalised structure and 0.125 for the localised (Huckel value zero). Core resonance integrals between neighbouring atoms, the analogues of Huckel's β, have theoretical values of −3.9 and −3.2 eV compared with −3.6 eV in benzene.