[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]

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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