[Pw_forum] where is the end of an unconverged NEB calculation

vega lew quantumdft at gmail.com
Sat Nov 7 07:45:18 CET 2009

Dear friends from CAS and Italy,

Thank you for your advice, I will try to adjust the parameters that you just
mentioned above.

At last, I found QE has been upgraded to 4.1.1, but the QE I used is 4.0.3.
I wonder whether the new vision of QE could do better than the older one.

Thank you again for your time.



On Fri, Nov 6, 2009 at 9:00 PM, Lorenzo Paulatto <paulatto at sissa.it> wrote:

> In data 06 novembre 2009 alle ore 04:35:39, vega lew
> <quantumdft at gmail.com> ha scritto:
> > During my NEB calculations, I found there are two possible situation.
> > First, the calculation could be converged after about 100 iterations.
> > Second, the calculation never converged in more than 300 iterations.
> Dear Vega,
> NEB calculation are a bit tricky in general and require extreme care to be
> setup correctly. NEB also takes easily hunders of iteration to converge,
> of course depending on the number of atoms and of images. Here is some
> free advice I can give, although only practice make prefection.
> 1. don't use climbing image at the beginning, CI makes convergence slower;
> especially if the special image changes during the convergence process (it
> may happen if CI_scheme='auto') this does ususally mess up everything.
> Converge your calculation than restart from the last configuration with
> the climbing image (note that this will *increase* the barrier!)
> 2. carefully choose the initial path, remember that QE assumens continuity
> between the first and last image at the initial condition. In other words,
> periodic images are NOT used, you may have to manually translate an atom
> by one or more unit cell base vectors in order to have a meaningful
> initial path.
> 3. you can try to start the NEB process with most of the atoms fixed in
> position, in order to have the more "problematic" one to converge, before
> freeing all of them.
> 4. especially for larger systems, you can start the NEB with lower
> accuracy (less k-points, lower cutoff) and then increase it when it has
> converged to refine your calculation.
> 5. use the Broyden algorithm instead of the default one, it is a bit more
> fragile, but it removes the oscillation problem. If this happens, and you
> cannot afford more images, focus to a smaller problem, decompose it to
> pieces.
> 6. you can grossly estimate the number of iterations required as
> number_of_images * number_of_atoms * 3 (the atoms that don't move should
> not be counted). It may take half that many iterations, or twice as many,
> but more or less that's the order of magnitude, unless you start from a
> very accurate or very bad initial guess.
> good luck
> --
> Lorenzo Paulatto
> SISSA  &  DEMOCRITOS (Trieste)
> phone: +39 040 3787 511
> skype: paulatz
> www:   http://people.sissa.it/~paulatto/<http://people.sissa.it/%7Epaulatto/>
>     *** save italian brains ***
>  http://saveitalianbrains.wordpress.com/
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Vega Lew ( weijia liu)
Graduate student
State Key Laboratory of Materials-oriented Chemical Engineering
College of Chemistry and Chemical Engineering
Nanjing University of Technology, 210009, Nanjing, Jiangsu, China
Email: vegalew at gmail.com
Office: Room A705, Technical Innovation Building, Xinmofan Road 5#, Nanjing,
Jiangsu, China
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