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[Nicola Seriani <seriani@tmfs.mpgfk.tu-dresden.de>]

  Good afternoon Prof. Finocchi,
               I would also be ineterested in receiving this article.

 Best regards
                         Nicola Seriani
                         Institute of Material Science
                         TU Dresden

On Thursday, May 27, 2004, at 19:12 Europe/Berlin, Fabio Finocchi wrote:

> Dear Conor,
> I read your email just now (!!!!). I hope you succeded in managing 
> polar slabs...
> In any case, there are few hints below.
> I have some experience in dealing with polar slabs. See, e.g. Bottin 
> et al, PRB 68 (2003) 035418, and references therein. I can also give
> you, on request, the text of an article that recently appeared in the 
> proceedings of a NATO-ASI school [F.Finocchi, F. Bottin and C.
> Noguera, "Computer simulations of polar oxide surfaces", in 
> Computational Materials Science, edited by C.R.A. Catlow and E. > Kotomin
> (NATO-ASI Series E, IOS Press, 2003), pages 196-217], where a detailed 
> analysis of the numerical problem is carried out.
> First of all, the physics: if your slab does not fulfill the electron 
> counting rule, it is likely metallic (but not necessarily, since some
> symmetry can be broken, which may open a small gap around EFermi). 
> Therefore, I would suggest you to use some smearing of the Fermi 
> surface,
> eventually using different values for tsmear (two are generally enough 
> to extract the T=0 total energy). Also, start a test calculation from
> a slightly perturbed configuration with a low symmetry.
> Second, the Broyden method is not very stable and can diverge whenever 
> the electronic structure is very sensitive to atomic relaxations. A
> quasi-continuum damped dynamics (ionmov=7, with equal atomic masses 
> amu~4, vis~10, and dtion~50) is better suited when atomic forces are
> less than, let's say, some mHa/bohr.
> Third, we have a home-made version of the code in which a compensating 
> dipole in the vacuum region is used. However, due to the lack of
> human time, nobody included it in a abinit version so far, to my 
> knowledge.
> Nevertheless, some quantities, such as the total energy, are not 
> extremely sensitive to the use of a compensating dipole, provided that 
> a
> reasonable vacuum region is added in the supercell. This is because 
> the electrostatic potential is mostly biased in the slab outer > regions,
> where the electron density is small.
> Others, like the work function, do actually depend on the use of a 
> dipole.
>
> Buona fortuna,
> Fabio
>
> cdhogan@roma2.infn.it a écrit :
>
> > Dear Abinites,
> >
> > Apologies in advance for the long post:  this will perhaps concern 
> > people with
> > experience in using slab geometries or with the internal workings of 
> > the
> > code.
> >
> > I'm trying to optimize the geometry of a polar slab: GaAs(001) [back 
> > surface
> > passivated by pseudohydrogens, Z=1.25], and I'm quickly running out of
> > options, as the calculation refuses to converge, at least with the 
> > Broyden
> > scheme.
> > The slab is ~11 layers thick, and I've tried
> > 7 and 11 layers of vacuum (the latter =15 Angs!). I'm using geometries
> > converged with another code to start, and the same pseudopotentials
> > (Hamann type from FHI code).
> > For odd reasons, Im using Abinit 4.0.3, compiled with IFCv7.1, on a 
> > Dell Xeon.
> >
> > Typical input parameters I've tried are:
> > ionmov 3 or 2
> > iscf 5 or 3
> > diemac 12 or 8
> > diemix 0.5 or 0.2
> > iprcell 45 or undefined
> > tolmxf 5e-4, toldff 5e-5 or lower
> > k-points: Gamma pt, 1/2/4 points in IBZ (1/4 SBZ)
> > The convergence of the SCF part is usually without problem, converging
> > in 10-20 steps.
> >
> > However, the total energy and forces, while initially decreasing,
> > jump to high values after a few steps
> > and oscillate; the atomic positions overshoot, and the system never
> > really seems to be relaxing smoothly - and even after this system has 
> > returned
> > to the original atomic coordinates the same instability occurs.
> >
> > I noticed that these jumps coincide with the one-dimensional 
> > electrostatic
> > potential showing a constant gradient in the vacuum (perp to 
> > surface), not to a
> > constant value. This I recognise as being associated with a 
> > macroscopic
> > electric field in the cell appearing due to the periodic boundary
> > conditions, which people have solved with dipole layers in the vacuum.
> > In the absence of such a correction, I would expect to see such a 
> > constant
> > gradient at all times, even for fixed geometry, but this does not 
> > occur.
> >
> > So my questions are:
> > 1) Can Abinit handle polar slabs?
> > 2) Does Abinit have, or plan to, incorporate such a dipole layer in 
> > the vacuum?
> > (or do I have to tackle it myself...?!)
> > 3) Has anyone managed to converge geometry for polar slabs, without 
> > just making the slab symmetric? (this would make my slabs too large)
> > 4) Has anyone any further suggestions or tricks for converging such
> > structures,
> > other than whats in the FAQ, etc.? Am I simply missing something?
> > It was suggested to me that it might be a problem with the pseudo...
> > 5) Where does the "sensitivity to the vacuum thickness" come from?
> > Would a molecular dynamics calculation help?
> >
> > Any comments, criticisms or full solutions (!) are most welcome.
> > All the best,
> > Conor
> >
> > ---
> > Dr. Conor Hogan
> > Dipartimento di Fisica
> > Universita' di Roma "Tor Vergata"
> > Tel: +39 06 72594548
> > Fax: +39 06 2023507
> >
> > What is it that makes a complete stranger dive into an icy river to 
> > save a
> > solid gold baby? Maybe we'll never know.  - Jack Handey
> >
> > To me, it's always a good idea to always carry two sacks of something 
> > when
> > you walk around. That way, if anybody says, "Hey, can you give me a
> > hand?," you can say, "Sorry, got these sacks." - Jack Handey
>
> --
>
>       Fabio Finocchi
>
>        Groupe de Physique des Solides  (UMR CNRS 7588)
>   Campus de Boucicaut, 140 rue de Lourmel, 75015 Paris (France)
>   tel: (33) 1 44275116    fax: (33) 1 43542878    bureau: 2.1.5