Transition state calculation for wealy bound molecules

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microomran
Posts: 10
Joined: Fri Aug 03, 2018 7:06 am

Transition state calculation for wealy bound molecules

Post by microomran » Wed Sep 12, 2018 3:27 am

Dear Dr.Graeme,

I am T.S. for different elementary reaction on Ni (111) .While most of calculation are going well and confirmed with one imaginary vibration,those of weakly bound molecules (namely CH4 and CO2) are not going well .

CH4:
After 150 iteration using QM:
0 0.000000 -200.038300 0.000000
1 0.076095 -200.030600 0.007700
2 0.218902 -197.373200 2.665100
3 0.262813 -195.285600 4.752700
4 0.157157 -195.424700 4.613600
5 0.217225 -195.084300 4.954000
6 0.252924 -197.042300 2.996000
7 0.412879 -197.557800 2.480500
8 0.176803 -197.794100 2.244200
9 0.009294 -197.983800 2.054500

Then switching to dimer (as below 0.5 eV) I got some positive curvature that persists after about 400 iteration:
Step Force Torque Energy Curvature Angle
1 0.99496 43.43601 -194.76306 22.99635 61.11114
1 0.99496 41.38738 -194.76306 -24.38334 46.97009
1 0.99496 8.24616 -194.76306 -18.51177 10.10026
1 0.99496 6.46426 -194.76306 -20.71544 3.24645
2 0.78132 7.54465 -195.38218 -3.71035 12.77097
2 0.78132 24.48226 -195.38218 -46.18370 36.89431
2 0.78132 13.16304 -195.38218 -32.24095 18.45483
2 0.78132 11.05970 -195.38218 10.83804 8.76179
....
20 1.07599 0.58094 -195.22214 0.05091 1.44860
21 1.16995 0.91533 -195.19354 0.07023 0.78856
22 1.23400 0.49295 -195.16451 0.07168 1.38721
23 1.32429 0.81208 -195.13735 0.05768 0.69044
24 1.37741 0.42034 -195.11009 0.02901 1.24705
25 1.45780 0.71129 -195.08484 0.03989 0.58221
26 1.50184 0.35152 -195.05947 0.06210 1.26089
27 1.57108 0.63423 -195.03676 0.06528 0.51063

Similarly with CO2 ,I have even got high forces when starting calculation that may not converge (after 50 iteration QD) :
0 0.000000 -199.171500 0.000000
1 16.472382 -197.844800 1.326700
2 0.809860 -198.928400 0.243100
3 2.732337 -194.419200 4.752300
4 6.463525 -193.745700 5.425800
5 7.247404 -194.691200 4.480300
6 7.349327 -195.932400 3.239100
7 7.528407 -196.360200 2.811300
8 7.612267 -196.832200 2.339300
9 0.009683 -197.728900 1.442600

For both cases I tried to repeat initial co-adsorption calculation but no significant change.I attached the INCAR files for both cNEB and dimer.
My current approach is to reduce forces <0.5 eV using QM and then switch to dimer.It worked with all other species.
Any advice to adjust those calculations?

Thank you in advance
Attachments
DIMCAR.txt
(3.79 KiB) Downloaded 2 times

graeme
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Re: Transition state calculation for wealy bound molecules

Post by graeme » Thu Sep 13, 2018 1:15 am

Before getting into the dimer calculations, the NEB calculations do not looks right. I see jumps of over 2 eV between images in the first calculation and oscillations in the energy and high forces in the second. Check these paths to see if they are reasonable. I would not be surprised if you are looking at molecular dissociation in the gas phase. I don't have the data to look at the pathways, but there is some problem with the paths even before you are starting the dimer calculations.

microomran
Posts: 10
Joined: Fri Aug 03, 2018 7:06 am

Re: Transition state calculation for wealy bound molecules

Post by microomran » Thu Sep 13, 2018 6:47 am

Hi Dr.Graeme

Yes,as you expected it is gas dissociation.Specifically ,I am facing the problem with H2,CH4 and CO2 which are physically adsorbed with week adsorption energies(-0.02 to -0.06 eV).The two main problems -as you mentioned- are either I got is either jump between image (+2eV) or high forces after my 1st 50 iterations using QM/cNEB.I tried to repeat co-adsorption in case of CO2 dissociation 2 times and start by QM again but forces still high.

The issue that I have used the same INCAR setting /methodology with all other molecules and it worked well ,confirmed by only 1 imaginary vibration frequency.Should I repeat Coadsorption again and redo cNEB? ; Can I reduce the number of images that I usually use (8 images) for the purpose of troubleshooting?

I attached that the data for your kind advice.

Thanks
Attachments
H_H_discn.zip
(76.86 MiB) Downloaded 1 time
CO_3rdtrial.zip
CO2
(5.61 MiB) Downloaded 1 time
CH4_trial.zip
(27.46 MiB) Downloaded 1 time

graeme
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Posts: 1752
Joined: Tue Apr 26, 2005 4:25 am
Location: University of Texas at Austin
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Re: Transition state calculation for wealy bound molecules

Post by graeme » Thu Sep 13, 2018 12:19 pm

The problem is your initial paths. You have mechanisms in which molecules are being broken in the gas phase and then a fragment adsorbs on the surface (in the case of CO2 and CH4) or just left in gas phase (H2). If you are trying to understand dissociative adsorption, you need to find stable final states for the products on the surface. To give some examples, you can look at publications #6 and #24 on http://theory.cm.utexas.edu/henkelman/pubs/ .

microomran
Posts: 10
Joined: Fri Aug 03, 2018 7:06 am

Re: Transition state calculation for wealy bound molecules

Post by microomran » Thu Sep 13, 2018 6:14 pm

Dear Dr.Graeme,

Thanks for your reply and share those two similar examples.One main idea-as you said- I got is that my final states are not stable on surface ((although they are converged).I will try fix that to see how it will work.

Thanks

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