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

[Giuseppe Mattioli]

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,
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