AbstractStationary points of the INDO energy hypersurface for various orientations of ethylene in external electric fields of the strengthF=0, 2, 4, 6, 8 and 10 × 1010V m−1were found and their characteristics studied by the force constant matrix analysis. Energies, structural parameters, charges,Wibergindices and dipole moments are presented. The only stable orientation of the ethylene molecule is that for which the CC bond is parallel to the field direction up toF=6 × 1010V m−1(orientation (a) inFig. 1). Above this value the molecule is structurally unstable and it decomposes to the hydride anion and the C2H3+cation. Rotational instability was found for two perpendicular orientations of the CC bond with respect to the field vector, in which the field vector was parallel and perpendicular to the molecular plane. Pseudorotations with negative eigenvalues of force constant matrices lead to the stable orientation (a). No stationary points were found when the angle between the CC bond and the field vector was between 0 and 90°. The five longest wavelength vibrational bands are presented for selected orientations and fie