[Pw_forum] Problem with relaxing large system

[Giovanni Cantele]

I’m not expert at all about Effective Screening Medium Method. However, checking with the documentation (PW/Doc/INPUT_PW.txt) I can see a first issue (I do not know how much related with your convergence problems): Requires cell with a_3 lattice vector along z, normal to the xy plane, with the slab centered around z=0. In your input file around z=0 I find an H2O layer, are you sure that this is what "slab centered around z=0” is meant in the documentation?

Second: by opening the output with XCrysDen I see an Al slab with an H2O layer on top, than at a distance of about 5 A along the z direction another H2O layer and, finally, > 5 A ahead along z two more H2O layers. Is this distribution of H2O layer what you need? I would say that too much spacing between different layers can cause convergence problems.

Third: a charge density issue might be showing up. Indeed, you are using two ultrasoft pseudo potentials (O.pbe-rrkjus.UPF, H.pbe-rrkjus.UPF) and one norm conserving pseudo potential (Al.pbe-rrkj.UPF). While the kinetic-energy cutoff (40 Ry) might be insufficient to get converged results with the norm-conserving pseudo potential, it is very likely that the charge density cutoff that you use (default value, 160 Ry = 4*40 Ry) produces approximate results of the charge densities coming from the ultrasoft pseudo potentials. The default value is indeed suitable for norm conserving pesudopotentials (for which increasing it is useless), but for ultrasoft pseudo potentials it should be set to 6 to 12 times (depending on the system) the kinetic energy cutoff (240 to 480 Ry in your case, provided that 40 Ry is a good value). Did you check the convergence of simple properties with respect to the two cutoffs before starting the more tricky calculation with the ESM method? Usually, step by step calculations always produce less waste of time than straightly carrying out the final calculation we need!


Giovanni



> On 17 Jul 2015, at 09:17, Nkosinathi Malaza <nuttymalaza at gmail.com> wrote:
> 
> Dear All,
> 
> I need assistance with relaxing the system of Al and H2O, the system has 123 atoms with ESM=bc3. The input file runs until it crashes, what can be the problem?
> 
> I have attached the output file.
> 
> Kind regards
> 
> N. Malaza
> Wits University
> Johannesburg 
> South Africa
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-- 

Giovanni Cantele, PhD
CNR-SPIN
c/o Dipartimento di Fisica
Universita' di Napoli "Federico II"
Complesso Universitario M. S. Angelo - Ed. 6
Via Cintia, I-80126, Napoli, Italy
e-mail: giovanni.cantele at spin.cnr.it
Phone: +39 081 676910
Skype contact: giocan74

ResearcherID: http://www.researcherid.com/rid/A-1951-2009
Web page: http://people.na.infn.it/~cantele