I am performing DFT-calculations (gamess-us) on a reaction: ground state 1 -> transitition state -> ground state 2. (in short: gs1 -> ts -> gs2.)
ts is only 0.3 kcal/mol higher in electronic energy than gs2.
In order to get the Gibbs energy, i added the Gibbs-correction (G) terms obtained from frequency analysis to the electronic energy of ts and gs2, respectively.
G for ts is +63 kcal/mol
G for gs2 is +64 kcal/mol.
Thus, the Gibbs-Energy of gs2 is 0.7 kcal/mol higher than the Gibbs-energy of ts.
How to interpret this? Does this mean that gs2 is not "real"?
Does it make sense to use gs2 as a starting point for further reaction mechanisms?
Can i use eyring equations to interpret ts and gs2?
Thank you in advance!
Thank you for your suggestions!
In fact, i have calculated this reaction with several basis sets, i have changed to triple-zeta, (6-311G(d,p), diffuse functions, i have tried MP2 - nothing helps. Of course i calculated IRC, and my lebedev grids are higher than default.
I tried calculating solvent -influence via PCM and by adding explicitly solvent molecules, the situation stays the same.
The overall energy change is not +64 kcal/mol, since gs2 also hast +63 kcal/mol as Gibbs-correction (see posting above).
Unfortunately, i cannot afford more than MP2 with my home pc! :)
The problem is, i think, the following: one should find a minimum on the Potential Gibbs-Energy Surface instead of the Potential Electronic Energy surface - if there exists such a minimum. Therefore, a lot of frequency calculations would be necessary - maybe several hundred.
And if such a state ( a Gibbs-minimum) could be found: How to interpret this state, if it is not simultaneously a minimum on the PES?
I have come to the conclusion that gs2 is a classical example for "thermodynamically impossible".
Maybe temperature changing would help - but in my case it is predefined.
Kind regards, Georg
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