A semi-empirical SCF molecular orbital method is developed in order to calculate rotational barriers and equilibrium conformations in the excited states of conjugated molecules. The new theory differs from the Pariser-Parr-Pople treatment in that differential overlap is neglected only when the atomic orbitals involved are not bonded to each other. Heats of formation and singlet excitation energies calculated with the modified theory agree well with the experimental values for a series of hydrocarbons and carbonyl compounds. The activation energy required for cis-trans isomerization in ethylenes is calculated as 62·3 kcal/mole, in excellent agreement with the experimental result of 65·0. Similarly, the energies of the planar and twisted forms of the lowest triplet of ethylene agree well with those obtained by the non-empirical calculations of Kaldor and Shavitt.