The ABINIT Users Mailing List ( CLOSED )

[cdhogan@roma2.infn.it]

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

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