Yes, there is; depending on your selection, Gaussian may use the Moller-Plesset fourth order correlation energy correction with single, double, triple and quaduple substitutions.

Dear Noel,

Thank you for your message. I understant that your there is means "there is a difference".  Could you please provide me with literature (references) explaining the difference?

Thanks in advance! Teodoro

YES, there is n definite difference. What order is this reaction A -> B ?

Dear Johan,

Suppose I have a reaction  in which my starting material is A and I get B. The transition state of the reaction is TS.  I can use Gaussian to calculate the energies of A, B and TS and get the corresponding energy differences (ETS-EA= DA and ETS-EB= DB; I can also write EB-EA= DAB). I can also determine the rate of conversión of A into B at two different temperaturas and apply the Arrhenius equation to measure Ea of "the process". My question is: Shall Ea be similar to DA, DB, DAB or none of them?

What will be the difference (i.e., what do they measure exactly) between Ea and DA, DB or DAB?

Thank you for your consideration. Literature references to read about are also welcome!

Sincerely

Teodoro

The value you get from Arrhenius will be close to the value of DAB. NOT ETS !!

Remember the transition state will be at a "higher" energy "level" than either the reactants or the products. Bonds are busy breaking and new bonds are forming and so the atoms in the valence level is in a very "exited" stage . This is the reason why catalyst will be needed in some cases to enable a larger amount of molecules, with weaker bonds because of the catalyst, to participate in the reaction. Less energy will be needed to reach the TS state.

Arrhenius's equation calculates the ACTIVATION ENERGY for the REACTION to take place.

Dear Johan,

Thanks for the explanation.Do you have any literatura (just the references) to read more about this specific topic?

Thanks a lot in advance

Teodoro