[Pw_forum] Two questions on a La2O3 slab: (1) inconsistent energy results; (2) "maximum number of steps has been reached"

[noon at seas.ucla.edu]

Apologies. Beginning user and first time post. My name is Daniel and my
affiliation is University of California Los Angeles (UCLA). Specifically, I
am a Ph.D. candidate in the chemical engineering department at UCLA.

The simulations for the La2O3 slab with a surface O-atom removed are all
identical (same 39 atom cell, same pseudopotentials, etc), except for
differing in one or more of the following: degauss, etot_conv_thr,
forc_conv_thr, mixing_ndim, upscale. The final energy results can vary
widely, like between -8330 Ry to -8360 Ry, far too large of a distribution
for the purposes the results are being used for. No such phenomenon happens
when that O-atom is kept intact, energies consistent to within about 0.5
Ry. I get the "maximum number of steps reached" issue (possibly non-issue?)
on all my simulations, O-atom intact or removed.

I have a sample input file attached (converted from UNIX text to text
format viewable in Windows). Output file was too big; not sure what samples
to take from it that would help aside from previous general descriptions of
what's going on.

Thanks for any help you may have to offer.

--
Daniel Noon
UCLA Chemical and Biomolecular engineering


On Fri, May 9, 2014 at 8:52 AM, Giuseppe Mattioli <
giuseppe.mattioli at ism.cnr.it> wrote:

>
> Dear unknown user
> First of all, please sign always your posts to this forum with your name
> and scientific affiliation, we care about it :-)
>
> Regarding your questions, it is impossible to help you without seeing (at
> least the relevant parts of) your input files. What do you mean when you say
> "separate simulations" of the same system? Did you change the cell or the
> vacuum region? Did you double the slab? Did you use different
> pseudopotentials?
> The second question can be also related to inconsistencies or errors in
> your input files. Again, it is impossible to give you advice without seeing
> them
>
> HTH
>
> Giuseppe
>
> On Friday 09 May 2014 07:57:20 noon at seas.ucla.edu wrote:
> > Hello,
> >
> > I've been trying out pw.x, with my first goal being to reproduce some of:
> > http://pubs.acs.org/doi/abs/10.1021/jp103604b. I've been running into
> some
> > issues and was wondering if I could get some advice:
> >
> > (1) When I run simulations of a La2O3 slab with a surface oxygen atom
> > removed (39 atom supercell), I get wildly inconsistent results
> (internally,
> > on QE, when doing multiple simulations) for the energy when doing
> "relax",
> > with a range of about 30 Ry. The distribution of resulting energies from
> > separate simulations seems to narrow when I tighten conv_thr, but I
> cannot
> > seem to get them to be acceptably consistent. Anyone familiar with this
> > issue and how to address it? I appear to have no such problem when there
> is
> > no oxygen atom removed, with energies consistent within about 0.5 Ry.
> >
> > (2) For all of my simulations of La2O3 slabs, regardless of what my
> inputs
> > are and as long as I do not run into the maximum number of iterations for
> > SCF or crash for another reason, I always eventually get the message "The
> > maximum number of steps has been reached" with the simulation stopping.
> > This message pops up even when it appears the number of SCF cycles and
> BFGS
> > steps do not reach nsteps. I notice that, during the simulations, it
> seems
> > to get to a point where the energy switches back and forth between two
> > values between SCF cycles, and then eventually it decides to stop with
> that
> > message. Is there a fix for this? Or is this something I don't need to
> even
> > worry about (i.e. maybe the final result is acceptable)?
> >
> > Appreciate any advice you may have to offer. Can provide more information
> > upon request. Thanks.
>
>
> ********************************************************
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>    Giuseppe Mattioli
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