Wrong Arrhenius plot

[Xenhua Zhang]

Dear Developers/ Users of TAMkin,

Thank you so much for an excellent tool such as TAMkin.

I have a very simple question, which I hope someone will answer to me.

I am using TAMkin to plot the Arrhenius plot of kinetic reaction. My reaction has well converged Initial, Transition and Final states. 

I am getting an intuitively wrong Arrhenius plot, with rateconstant increasing as temperature decreases.

I know that I am doing something very stupid here. Could someone please point me what I might be doing wrong here.

Thank you in advance.

Xenhua

[Toon Verstraelen]

Hi Xenhau,

This can indeed happen in some cases. Usually this is an indication of incorrect inputs, but it is hard to tell from the figure alone. Can you email me a zip file with a working example resulting in this figure? That should make it possible to tell what is going wrong.

Best regards,

Toon

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Prof. Dr. ir. Toon Verstraelen

Center for Molecular Modeling

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[Xenhua Zhang]

Dear Toon,

Thank you for your reply.

Please find attached my TAMkin sample folder, where I get one of such result. Basically, I am using the script "tamkin-driver" which is bundled with the package.

I will send you my native vasp files in a separate email so that you can verify that the calculated IS, TS and FS are exactly the states I am using with TAMkin.

I greatly appreciate your time and any advice about my setup.

Best regards,

Xenhua

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[Toon Verstraelen]

Dear Xenhau,

I'm replying back on the list after taking a look at your files, because it might be a useful hint in general. The problem is that KS-DFT energy of your transition state is lower than that of the reactant. You can see this in the file kinetics.txt:

Electronic energy barrier [kJ/mol] = -82.5

This negative enthalpic contribution to your activation energy is the leading term and results in a negative activation energy as well. This in turn leads to the somewhat odd Arrhenius plot. The solution is to check why the enthalpic barrier is negative, which is essentially the result of your VASP calculations.

Best regards,

Toon

On Mon, May 20, 2019 at 3:12 PM Xenhua Zhang <kmc.c...@gmail.com> wrote:

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[Xenhua Zhang]

Dear Prof.,

Thank you so much for your time and efforts to investigate my problem.

May I ask you, Does " Electronic energy barrier [kJ/mol] = " printed in kinetics.txt have any other contributions, such as ZPE contributions or entropic contributions included in it?

I ask this because, during my Vasp NEB calculation analysis, my nebef.dat (from vtsttools) was pretty normal and did not raise any red flags (like negative energy barrier etc). My nebef (with forces and energies) was 

                                          (energy)

   0         0.005506      -208.204900         0.000000 (IS)

   1         0.009309      -208.012100         0.192800 (TS)

   2         0.009776      -209.510400        -1.305500 (FS)

As you can see, the barrier is very low, only <0.2 eV. If any contributions are added to the barrier, there are very good chances that the barrier goes into negative.

Thank you again.

Best regards,

Xenhua

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[Toon Verstraelen]

Dear Xenhau,

That is indeed with electronic energy means here. I'm not a regular VASP user, so I cannot give useful comments on the VASP calculations.

Best regards,

Toon

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[Xenhua Zhang]

Dear Prof. Toon,

Thank you so much for your time and advice.

I have worked around this issue by writing the original reaction in reverse. That is, changing the original IS to FS and the original FS to IS. Doing this gives me a correct arrhenius plot.

The only thing is I will then have to consider the calculated forward rate constant as reverse rate constant and vice versa. 

Best regards,

Xenhua

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