## converge slowly

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### converge slowly

Hi, evergyone.

I am now calculating CH4 dissociation on Ni(100).When I use the CI-NEB method to find the MEP, the calculation is converged very slowly.

Initially, I use the NEB method without Climbing Image and I use IBRION=3 and POTIM=0.01 for several steps (e.g. 5). Then I copy CONTCAR to POSCAR, and use IBRION=1, POTIM=0.1 to converge the maxmum force in every degree of freedom to within 0.01 eV/A. Finally, the CI-NEB method is activated and IBRION=1, POTIM =0.1 are still used. It always took me several hundred steps to converge the calculation.

I wonder what is wrong with my calcultion procedure and if there is some skill using CI-NEB method.

Thanks.

I am now calculating CH4 dissociation on Ni(100).When I use the CI-NEB method to find the MEP, the calculation is converged very slowly.

Initially, I use the NEB method without Climbing Image and I use IBRION=3 and POTIM=0.01 for several steps (e.g. 5). Then I copy CONTCAR to POSCAR, and use IBRION=1, POTIM=0.1 to converge the maxmum force in every degree of freedom to within 0.01 eV/A. Finally, the CI-NEB method is activated and IBRION=1, POTIM =0.1 are still used. It always took me several hundred steps to converge the calculation.

I wonder what is wrong with my calcultion procedure and if there is some skill using CI-NEB method.

Thanks.

Also, if you are willing to try a new method, we would be very interested to see if a double nudging scheme (from Wales' group) improves the convergence using IBRION=1. You can get an updated neb.F with this modification from cvs using the command

cvs -d :pserver:anonymous@theory.cm.utexas.edu:/Groups/cvs login

cvs checkout vtsttools

and recompile vasp. Use the password 'anonymous' when prompted. You can turn on the double nudging by adding the following flags to the INCAR file:

LDNEB=.TRUE.

LDNEBNEW=.TRUE.

The second flag turns on a modified version of the double nudging which we've found to work better with bfgs (ibrion=1). If you find that this improves convergence times for your CH4/Ni(100) run, we would be interested to know about it.

http://theory.cm.utexas.edu/vtsttools/code/

There are also any number of free programs that allow you to access cvs servers through windows, both graphical and from the command line.

LCLIMB=.TRUE.

It would be interesting to start your calculation after you did a few IBRION=3 steps, and compare how long it takes to converge using the climbing image and double nudging, with your original calculation.

I have tried the new method. Now, the calculation still can not converge after sixty ionic steps. I have used nebef.pl and the results are:

1 0.01753400 -254.52972900 0.01544300

2 0.03279600 -254.48991500 0.05525700

3 0.06137800 -254.37285500 0.17231700

4 0.13588800 -253.65668600 0.88848600

5 0.05486500 -254.05353600 0.49163600

6 0.02485600 -254.29669500 0.24847700

I think the POTIM=0.35 is not too large according to the forces, right?

I know you have calculated the Ni(111)/CH4 system and can you share your experience with me?

vfin.pl dir

cd dir

nebresults.pl

which will archive your run into the directory dir and then do a force based spline along the minimum energy path to give a better estimation of the barrier. Using the climbing image will also give you a very good estimation of the energy barrier because the highest energy image will be at the saddle point when the band converges.

If you want to make sure the entire band is converged, including this problamatic image, I suggest using even more conservative parameters. The quadratic optimizers (ibrion=1 and 2) tend to perform poorly near the minimum if the forces are not accurate enough. To improve robust convergence, you can use IBRION=3 and POTIM=0.1, or choose a smaller value of ediff (1e-6 for example), which will give you more accurate forces and better convergence using IBRION=1.

In fact, I find the reason why the forces on image 02 can not be converged to 0.01 ev/A is that the initial guess may be far away from MEP. If you can see my CONTCAR in directory 00~04, you can find in image 00, 03 and 04, the two H atoms which are far away from the Ni surface are in a plane parallel to the Ni surface, while in image 01 and 02, the plane with this two H atoms involved are not parrllel to the Ni surface.

So, the problem is that how to optimize the CH4 orientation in 01 and 02 images. My present POTIM seems to be too small to change the CH4 configuration for such a large extent, right?

Best