Mark Thompson suggest that a lack of rotational invariance might account for the descrepancy. Nice try, but that's not it. The difference between using the standard and z-matrix orientations in Gaussian 92 is 5x10^-7 kcal/mole. Similarly, using two different orientations in MOPAC 6.0 produces no change in the 5 decimal places printed in the energy. Mike Frisch -------