The ABINIT Users Mailing List ( CLOSED )

[Steven Homolya <Steven.Homolya@spme.monash.edu.au>]

On Thu, 9 Oct 2003, Allan, Douglas C Dr wrote:

> Another and perhaps better way to select compatible k-points is to
> start with a single k-point in the supercell (since it is a supercell
> a grid of multiple k-points does not really make sense) and then do
> zone-folding to make a set of k-points in the primitive cell.  In the
> absence of the vacancy in the supercell this should produce the same
> energy/atom in both cases to very good accuracy; they results should
> agree within the tolerance you supply for convergence.  Then when you
> put the vacancy in the supercell you can better trust the energy
> comparisons between the two cells.
> 

I'm still sorting k-mesh compatibility out for the systems I'm looking at, 
so I'm happy to read your suggestions.

My questions are:

* Why shouldn't one use multiple k-points for a supercell?
* How does one "unfold" the B.Z.?

I found that one k-point is not enough for 16, 27, 32, 64, or even 108
site fcc supercells (when I want at least 0.0005 Ha accuracy in total
energy).

E.g. fcc-Cu:

- happy with 1-atom prim. cell using ngkpt 16 16 16
- get the same result using 8 atom fcc supercell with ngkpt 8 8 8
- get the same using 64 atom fcc supercell with ngkpt 4 4 4

If I use only one k-point for 64 atom supercell, that would equate to
4x4x4 == 64 k-points in the B.Z. of the one atom fcc unit cell, which is
insufficient for anything but a rough initial estimate of optimal
geometry.

Steve

-- 
Steven Homolya
School of Physics and Materials Engineering
Monash University, VIC 3800
Australia
Tel: +61 3 9905 3694
Fax: +61 3 9905 3637