I have an IRC graph for one of the TS structure and it will give both reactant and product on either end of the IRC. I got some value after considering the energy difference b/n reactant and TS. But if I take the same reactant structure obtained by IRC and optimized again and then take the energy difference b/n this optimized structure and TS, the energy difference will be more. In such case which energy difference should be considered - that obtained by IRC or the optimized reactant and TS structure?
Energy difference or Energy Barrier:
IRC energy (6 kcal) < optimized reactant and TS (38 kcal)
G-09 package (B3LYP/6-31g, 300K, implicit water)
Hi Chethan,
If these two geometries are similar then the one you optimized later is the true minimum and that is the energy you should use. When you do an IRC search, you will usually also have the zero point energy values at the minimum and the TS. For a more accurate result, you can use the difference in these two ZPE values to correct your barrier height. TS has one less vibrational mode that contribute to ZPE, so this correction will make your barrier height lower.
You should also check your output to see if your IRC calculation actually reached the minimum. By default, Gaussian will stop the IRC calculation after 20 steps, whether it finds a minimum or not. If Gaussian stops after it finds a minimum, optimization from that geometry should converge at the first iteration, and the energy should not change at all.
In case Gaussian terminates after a max number of steps, and you would like to finish the IRC path, you can use the "restart" option to continue IRC the previous search; or you can use "MaxCycle=N" syntax to change the default step limit.
Dear Liu
Thank you for your reply. As i said above the energy barrier b/n reactant to TS is 6 kcal if i consider the IRC results and if i optimized the same reactant structure, then the energy barrier will be 38kcal with respect to TS structure.
Dear Zhu
Thanks for your kind response. As you mentioned IRC considered only 21 steps and gave 2 local minima representing product and reactant structures on either side.
Hi,
What is the purpose of your IRC calculation ? Is it to confirm the nature of the TS you get ? ie that it connects the reactant to the products ?
In any case, as Xiaolei said, Gaussian usually does not give you the real structure of the reactants or products. Starting for the structure obtained at the end of the IRC, you HAVE to re-optimize them to get the real reactant and product geometries and energies. So the larger energy is surely the correct one in your case.
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