The ABINIT Users Mailing List ( CLOSED )

[Igor Lukacevic <ilukacevic@fizika.unios.hr>]

Dear Matthieu,

Thank you for the further clarifications.

> You shouldn't be fixing the atoms for the first dataset: it probably
> does nothing as is. Further, given the high symmetry of the xred, it
> probably won't move the atoms anyway (if it should you might try less
> symmetric positions: this might be keeping abinit from finding the bct
> phase).

You're right, this is not needed.

> I notice you are looking for a high-pressure phase. Is the 12.5GPa
> from experiment? It could be that your system is not even metastable
> wrt cubic Si at that pressure (in DFT) and that you need a higher
> pressure.

Good idea, but I think it isn't the case, because I used LDA. And LDA gives 
the transition pressure cd -> beta-tin at about 7-8 GPa. If I used GGA, which 
raises the transition pressure up to 12-13 GPa, then this could be the 
reason. Experimentally beta-tin is stable in pressure interval 11.4-13.5 GPa.

> If you start from a non-cubic cell, and abinit makes it cubic, that
> should be the right result (for your pseudos kpoints etc... of
> course): it will not impose more symmetry, but should enforce the
> initial one. What if you re-enter your final geometry into a new
> input, does abinit find a different space group?

You're again right. I think that Josef explained my error (not abinits) or 
better to say shortsightedness.

> your tsmear is quite large, but with cold smearing this should be ok.
> I would go down to 0.02 at most.

I have not tested yet any of the convergence parameters, but thanks for the 
advice.


Thank you again!

Igor



> On Wed, Jul 15, 2009 at 12:32 PM, Josef Zwanziger<jzwanzig@gmail.com> wrote:
> > Dear Igor,
> >
> > when I run v5/t37 all the way to the end (upping ntime from the value
> > 2, which is used in the tests just to keep it short), it converges to
> > a BC tetragonal cell, like it should: here's the final cell geometry-
> >
> > Unit cell characteristics :
> > acell=  8.4892915048E+00  8.4892915048E+00  8.4892915048E+00
> > rprim= -6.5662241853E-01  6.5662241853E-01  3.7107142032E-01
> >           6.5662241853E-01 -6.5662241853E-01  3.7107142032E-01
> >           6.5662241853E-01  6.5662241853E-01 -3.7107142032E-01
> >   ucvol=  3.9152838280E+02 Bohr^3
> >   lengths=  8.4892915048E+00  8.4892915048E+00  8.4892915048E+00 Bohr
> >   angles (23,13,12)=  97.91442980  97.91442980 136.43658054 degrees
> >
> > this is done with the PAW dataset provided by the test suite. What
> > pseudopotential are you using, and cell geometry are you getting?
> >
> > Joe
> >
> >
> > On Wed, Jul 15, 2009 at 6:54 AM, Igor
> >
> > Lukacevic<ilukacevic@fizika.unios.hr> wrote:
> >> Dear abinit users,
> >>
> >> I am having some problems while trying to relax the body centered
> >> tetragonal phase of Si (beta-tin) using primitive unit cell with
> >> optcell=2.
> >>
> >> First I have tried with abinit-5.4.4p version, which stopped the
> >> calculation usually in the 1st or 2nd broyden step of dataset 2 (when
> >> cell size began to change), saying that the rprim change is too large,
> >> while I knew that it could not be. Whatever I put for dilatmx and
> >> starting aproximation of acell did not help. I did notice that when
> >> dataset 2 starts, it does not start with the acell I gave it through the
> >> input file (a=b and c), but it changed the acell values to the ones
> >> corresponding to the cubic cell (a=b=c). How come that this happens?
> >>
> >> Then I saw that newer versions of abinit 5.7 and 5.8 have a bug fixed
> >> concerning the symmetry preservation during relaxation in body centered
> >> tetragonal cells when one uses optcell=2 (test v5, t37). Using 5.8.3
> >> version indeed made the calculation continue and finish, but the final
> >> result is rather confusing. I always get a cubic cell in the end. This
> >> is the analogous result to the one in t37.out from v5 test.
> >>
> >> I would like to know why this happens and how come that I do not get a
> >> relaxed tetragonal unit cell in the end?
> >>
> >> The same things happen with a body centered orthorhombic unit cell of Si
> >> Imma phase.
> >>
> >> Using nonprimitive unit cell does not have this problem (but the
> >> calculation is ofcourse longer).
> >>
> >> The calculations are tried on 3 different computers with versions 5.4.4p
> >> and 5.8.3.
> >>
> >> Input file is attached (the parameters are not at their converged
> >> values, but I think that does not influence the issue).
> >>
> >>
> >> Thank you all in advance for your help!
> >>
> >> Yours Igor Lukacevic
> >>
> >> --
> >> Igor Lukacevic
> >> Department of Physics
> >> University of J. J. Strossmayer
> >> Trg Ljudevita Gaja 6
> >> 31000 Osijek
> >> Croatia
> >
> > --
> > Josef W. Zwanziger
> > Professor of Chemistry
> > Canada Research Chair in NMR Studies of Materials
> > Director, Nuclear Magnetic Resonance Research Resource (NMR-3)
> > Dalhousie University
> > Halifax, NS B3H 4J3 Canada
> > tel: +1 (902) 494-1960
> > fax: +1 (902) 494-1867
> > web: http://jwz.chem.dal.ca
> > jzwanzig@gmail.com, jzwanzig@dal.ca

-- 
Igor Lukacevic
Department of Physics
University of J. J. Strossmayer
Trg Ljudevita Gaja 6
31000 Osijek
Croatia