[Pw_forum] where is the end of an unconverged NEB calculation
paulatto at sissa.it
Fri Nov 6 14:00:03 CET 2009
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.
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
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
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.
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