Hi, I conducted SN2 reaction transition state determination in gaussian. I calculated the activation energy of the reaction by using the free energy I obtained in the output file. (Free energy (TS) - Free energy (reactant))
However, my panel wanted me to incorporate the thermal corrections and zero point energy, single point energy, and other energies.
How do I solve for the activation energy using that approach?
Thank you!
Hi Trisha M Castilla ,
You have to perform a frequency calculation on the transition state, verify that it has only one imaginary frequency, which connects starting and end state.
From the results of the frequency calculation, you obtained the zero point correction or, including the temperature, the gibs free energy correction, which can be added up to the single point energy.
G(298K) = E_tot (HF/DFT/CCSD energy) + G(RRHO,298) (from frequency calculation)
U = E_tot + E_ZPVE for inner energy at 0K
Some more infos and theory can be found here.
https://gaussian.com/thermo/
It is usually recommended to calculate the correct electronic structure energy at a higher level of theory (Double Hybrid DFT, CCSD(T)) and the thermal correction at a lower level of theory. I think it is a good idea to avoid hybrid functionals for frequency calculations as it is quite time consuming and depending on the basis set and HF part, the frequencies have to be scaled by a factor between 0.9 and 1.0.
Good luck and best regards,
Conrad
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