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

[Giuseppe Mattioli]

Dear Daniel
I do not know the properties of your La oxide but now I can try to  
give you some help. There are several things you should try to change  
in your input file, but first of all the plane-wave cutoff  
(ecutwfc=30) seems to be too low for your PAW pseudopotentials. Did  
you try to reach convergence on the basis set (e.g., all forces very  
similar at the end of the first scf iteration on the same geometry)?.  
Moreover, if you do not indicate the density cutoff (ecutrho), a value  
of ecutwfc*4 is automatically assigned. This is a good choice for  
norm-conserving pseudopotential, a poor choice for ultrasoft  
pseudopotential, and a choice that should be checked in the case of  
PAW pseudopotential (your case).

> degauss = 1D-4

Very low. A higher value of gaussian smearing (0.05~0.01) helps the  
scf to reach the ground state. If your defective oxide is an insulator  
(probably it is not...), the energetics also should be fine with 0.01,  
you can restart the relax calculation near to the final convergence  
with this lower value. If the defective oxide puts electrons in the  
conduction band (very common for oxygen vacancies in oxides), you may  
try with the Marzari-Vanderbilt ('mv') smearing.

You did not specify nspin, so nspin=1. This is probably a good choice  
for the intrinsic oxide, but what about the vacancy? You have two  
excess electrons (in a semiconductor slang), are they diamagnetic or  
paramagnetic? Forcing a diamagnetic unstable state could be a possible  
reason for the reported instabilities.

> assume_isolated = esm,
> esm_bc = bc1

Why? Try before to converge your system without such electrostatic  
tricks. I've never used the esm part of the code, and I'm not even  
sure that it works properly. It is not, for sure, something to begin  
with!

> conv_thr = 1D-7,

In difficult cases you may try to start with lower values, and even to  
use the scf_must_converge keyword (with care...) to recover from a  
problematic starting geometry.

> mixing_beta = 0.4,

Lower values (down to 0.05~0.01) help scf convergence.

nosym = .TRUE.,

Why? symmetry speeds up the calculation.

> mixing_mode = 'local-TF',
> mixing_ndim = 8
> upscale = 1D+3

not necessary, if everithing else has been setup correctly

K_POINTS (gamma)

Your supercell is rather small, that is, large in the reciprocal  
space. I do not know if you are familiar with reciprocal space  
techniques (if you are not, I recommend a certain degree of  
improvement of your knowledge...:-)), but you probably need to check  
the convergence wrt the k-point sampling of the Brillouin zone.
For example, use

K_POINTS (automatic)
n1, n2, n3, 0, 0, 0

and raise n1, n2, n3 until the forces/eigenvalues/.../what you're  
interested in is converged. Of course, the more the k-points the  
heavier the calculation...

> Maximum number of steps:
> electron_maxstep = 800,

Does the scf stop before reaching such a value? the nstep value (in  
&CONTROL list) must be assigned. the default value for relax  
calculations is 50. Does the bfgs procedure stop before reaching 50,  
complaining that "The maximum number of steps has been reached"? It  
would be an hapax, at least in my experience... By the way, which  
version of the code are you using?

When you are more skilled, you will probably consider to treat your  
defective oxide at a DFT+U level. We will speak again about it...

HTH and welcome to the QE community

Giuseppe

Giuseppe Mattioli
ISM-CNR
Italy

Quoting 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
>>    CNR - ISTITUTO DI STRUTTURA DELLA MATERIA
>>    v. Salaria Km 29,300 - C.P. 10

>>    I 00015 - Monterotondo Stazione (RM)
>>    Tel + 39 06 90672836 - Fax +39 06 90672316
>>    E-mail: <giuseppe.mattioli at ism.cnr.it>
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-- 
********************************************************
- Article premier - Les hommes naissent et demeurent
libres et ègaux en droits. Les distinctions sociales
ne peuvent être fondèes que sur l'utilitè commune
- Article 2 - Le but de toute association politique
est la conservation des droits naturels et
imprescriptibles de l'homme. Ces droits sont la libertè,
la propriètè, la sùretè et la rèsistance à l'oppression.
********************************************************

    Giuseppe Mattioli
    CNR - ISTITUTO DI STRUTTURA DELLA MATERIA
    v. Salaria Km 29,300 - C.P. 10
    I 00015 - Monterotondo Stazione (RM)
    Tel + 39 06 90672836 - Fax +39 06 90672316
    E-mail: <giuseppe.mattioli at ism.cnr.it>