Hi
I optimized all reactants and products. all frequencies and directions are correct but the delta G of reaction is positive. Delta G should be negative because the reaction is Automatically. I use B3LYP/6-31+G** and a have Se and S in my reactant and product
Dear Ramesh,
In addition to the explanation of Alrik, I would like to suggest you to take into account also the contribution of the Entropy (you can find it below Gibbs energy in output or also, you can calculate it with the general formula).
On the other hand, to see some differences with functionals, I attach a guide that it can help you. Also, via private or here, could you send an example of: from where and how you calculate DeltaG? A thing that you also can do it, it is calculating it via experimental values.
I hope it helps you,
Joaquim
Technical Report Gaussian Guide. Section 2. Optimization Methods. DFT, MP2 and CCSD.
Remember a positive free energy does not mean the reaction doesn't take place it means the reaction is not spontaneous under the given conditions and in the set direction. So if you're free energy is positive it simply means you reaction is not spontaneous in that direction.
Mrs/Miss Kheirabadi,
There is not obligatory the DG value of the product to be negative. The spontaneous processes are associated with a decreasing in DG. So that there are taken into consideration the DG values of the initial states (initial molecules) and corresponding intermediates.
Entropy: A concept that is not a physical quantity
https://www.researchgate.net/publication/230554936_Entropy_A_concept_that_is_not_a_physical_quantity
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