The ABINIT Users Mailing List ( CLOSED )

[mmikami@rc.m-kagaku.co.jp]

Hello (Buon giorno),

Just a few comments:
- ABINIT can treat the GaAs slab passivated with the fictious hydrogens.
  In my experience (long ago...so details in my tapes somewhere),
 I tried small slab models for (2x4)beta-1 and c(4x4).
 H(Z=0.75)-As-Ga-As-Ga-As for (2x4)beta-1,
 H(Z=0.75)-As-Ga-As-Ga-As-As for c(4x4).
The vacuum layers has about the same thickness of the slabs (or a bit 
thicker) .
# Caution ! This may not be sufficiently large models. But they may be 
good starts ...
# My goal was models with adsorbates, so I did not go into details for 
the larger models.
At that time, the geometry optimizations were OK without any tricky 
options.
(The options cited in the e-mail below will work ...)

I just suspected pseudopotentials ...
Have you checked the pseudopotentials of Ga/As/H ?
(As you would know well, some tricks will need to avoid ghost states ...
 e.g. Xavier Gonze et al, Phys.Rev.B44, 8503–8513 (1991)
 FHI98PP package has the functions to check the transferability of PPs
 as well as the estimation of needed ecut. (Other PP packages also will 
do ...))
In passing, how about enegy cutoff ?

In any case, disclosure of more details might be helpful for readers
to grasp (not to guess) your situation ...

Regards with "Buona fortuna",
Masayoshi

P.S.  I would appreciate the result of "ifc -V" "uname -a" on your 
Xenon machine.

On 2003.11.6, at 23:55 Asia/Tokyo, cdhogan@roma2.infn.it wrote:

> 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