[Pw_forum] Two questions on a La2O3 slab: (1) inconsistent energy results; (2) "maximum number of steps has been reached"
Giuseppe Mattioli
giuseppe.mattioli at ism.cnr.it
Tue May 13 10:59:42 CEST 2014
Quick answer: if you do not have a huge dipole moment in the z direction and you put enough vacuum between periodically repeated images of the slab
along z, you can perform a periodic calculation without using any electrostatic correction to the potential. This is what most people usually do.
There is also a post-processing tool which averages potentials along z (see yourQE/PP/examples/WorkFct_example). You may check that the calculated
average is flat in the middle of the vacuum region. In the case of a large dipole, you can also apply a sawtooth dipole correction (with the "tooth" in
the middle of the vacuum region), as detailed in yourQE/PP/examples/dipole_example.
HTH
Giuseppe
On Monday 12 May 2014 22:28:29 noon at seas.ucla.edu wrote:
> Thanks.
>
> I have one quick question for you on one of your comments that stands out
> to me. I read through the pw.x input file description and, from what I
> ascertained (possibly incorrectly), using assume_isolated = esm and esm_bc
> = bc1 is how you would have the program recognize your structure as a slab
> (two dimensional; periodic in x and y but not in z; vacuum on the faces
> normal to z-axis).
>
> Is there another way to model a slab? If not, I would have to use those
> settings and it sounds like that could be pretty problematic unfortunately.
> Having it set as a slab is critical (no periodicity in z-direction).
>
> Will get back to you on other remarks later. Thanks again.
>
>
> --
> Daniel Noon, UCLA Chemical Engineering
>
>
> On Mon, May 12, 2014 at 8:58 AM, Giuseppe Mattioli <
>
> giuseppe.mattioli at ism.cnr.it> wrote:
> > 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.
> > >>
> > >> ********************************************************
> > >> - 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,
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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>
> > >> ResearcherID: F-6308-2012
> > >>
> > >> _______________________________________________
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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>
> >
> > _______________________________________________
> > Pw_forum mailing list
> > Pw_forum at pwscf.org
> > http://pwscf.org/mailman/listinfo/pw_forum
********************************************************
- 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>
ResearcherID: F-6308-2012
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