Activation energy

["Duc Nguyen" <anhduc -AatT- umich.edu>]

Hi all,
 Normally, how would you estimate the intrinsic Arrhenius activation energy
 of a reaction pathway. Would you use:
 1/ the potential energy of the transition state
 2/ the difference in potential energy between the transition state and the
 reactants: (classical energy barrier)
 3/ the difference in zero-point energy between the transition state and the
 reactants: (quantum energy barrier)
 4/ the difference in heat of formation between the transition state and the
 reactants
 Laidler (Chemical Kinetics, 1987) showed the differents between the last 3
 and derived an equation to calculate the instrinsic Arrhenius activation
 energy from the difference in heat of formation between the transition state
 and the reactants as: Ea = deltaHf_transitionstate - deltaHf_reactants + RT
 (for reaction in solution and unimolecular gas phase reactions).
 How does MOPAC or other quantum mechanisc package calculate the potential
 energy and zero-point energy of a compound? How normally are they different?
 Does it vary from compounds to compounds?
 Thank you very much in advance,
 Regards,
 Duc