UT theoretical chemistry code forum

Hi.

I'm calculating the energy barrier of CO oxidation.
Although I found some posts about the appropriate number of images (3~5 or 4, 6, 8), I have a question.
My calculation shows that the energy barrier depends on the number of images. (I attached a graph about it)

I'm wondering that 4 images are enough or 9 to 10 images are required.
or is there any criteria on determination of number of images? (like, 0.01eV/image)

thanks.

I suggest minimizing the intermediate configuration and run two separate bands of 3-5 images for each elementary step.

[quote="graeme"]I suggest minimizing the intermediate configuration and run two separate bands of 3-5 images for each elementary step.[/quote]


Thanks for the quick reply.

But did you understood that the attached figure (and my question) is about energy barrier vs. reaction coordinate?
Actually the reaction is quite simple. O-O-C-O* -> O* + CO2.
So there is a only one energy barrier (one transition state).
The question was about energy barrier depending on "number of images" in the simple reaction.

Or did I misunderstand...? If it is, can you explain more in detail...?
Thanks.

I'm looking at the energy pathway that you posted. You may view this as a single mechanism, but there is clearly an intermediate minimum along the path. I suggest minimizing that image (the 5th point in your plot) to see what minimum that corresponds to. Then, calculate separate paths for each elementary step.

[quote="graeme"]I'm looking at the energy pathway that you posted. You may view this as a single mechanism, but there is clearly an intermediate minimum along the path. I suggest minimizing that image (the 5th point in your plot) to see what minimum that corresponds to. Then, calculate separate paths for each elementary step.[/quote]


Thank you for the response. And I'm sorry that I questioned vaguely.

The x-axis in the figure is the number of images used in INCAR file. So the figure I attached is not energy pathway. when you closely look at the y-axis, it is energy "barrier", not energy.
So, what I did is calculating energy barrier of same reaction with varying IMAGE-tag in INCAR.

Now, I can question again.
I wonder whether 4 images are enough or 9 to 10 images are required in my calculation.
or is there any criteria on determination of number of images in INCAR (IMAGE-tag)?

thanks.

Oh I see. You should use the climbing image method so that a single image will converge right to the saddle.

[quote="graeme"]Oh I see. You should use the climbing image method so that a single image will converge right to the saddle.[/quote]


Dear Dr. Henkelman,

Thanks for your response, again.

The thing is that I already used the climbing image method as you can see the OUTCAR file.

OPT: Using VASP Quick-Min optimizer
CHAIN: Read ICHAIN 0
CHAIN: Running the NEB
NEB: SPRING -5.000000
NEB: LCLIMB T
NEB: LTANGENTOLD F
NEB: LDNEB F
NEB: LDNEBORG F
NEB: LNEBCELL F
NEB: EFIRST 0.000000
NEB: ELAST 0.000000

So, I was kind of confused that if the clmbing image method can find the saddle point, why such results can occur.

If you could post two of these calculations which give different barriers, I should be able to see what's going on.

Dear Soon Ho Kwan and all,

I have same problem like you, I would like to share my results as well. I tried 3 different number of images; 5,10,20, respectively. If you focus my graphs, you can understand so easily.

I am confused that is there any limitation of number of images?

Many Thanks!

When you have a set of connected minima and saddles you are finished. By connected I mean that a steepest descent path along the negative mode (in both directions) from each saddles gives you a connected path from your initial state to final state. In your case, it appears that 5 is too few and 10 is sufficient, but the minimization check that I mentioned would help you be confident of that.