Several users have criticised the tolerances within MOPAC. The point made is that significantly different results have been obtained when different starting conditions have been used, even when the same conformer should have resulted. Of course, different results must be expected -- there will always be small differences -- nonetheless any differences should be small, e.g. heats of formation (HoF) differences should be less than about 0.1 kcal/mole. MOPAC has been modified to allow users to specify a much higher precision than the default when circumstances warrant it.