CCL: reaction barrier and the orbital gap

 Sent to CCL by: "Rina  Dao" []
 I am wondering about the relationship between the calculated activation energy
 and the (also calculated)energy gap of frontier orbitals of the substrate. For
 example, I have done some calculation on the LUMO orbital of one reactant and
 the HOMO of the other(they are the best energy match).  So, does the Ea have
 inner connection with the "gap"? Or, could  the Ea be regarded almost
 equal to
 the difference of energy of one molecule's LUMO and the other's HOMO ?