Hi Jafar,
I've done a convergence study for a SiC surface and Ni[100], but I guess
the general features will be the same, quite independently of the
material. The nice thing about siesta is that vacuum layers come virtually
for free, as opposed to plane-waves calculation.
So, maybe the first thing to do is to determine which is the minimum
vacuum layer thicness to have an isolated slab of gold. I guess 10 Ang
would already be a good choice, but of course this has to be checked. Once
I determined the optimum vacuum thickness, I determined the largest slab
thickness, and built the supercell for my largest slab thickness. The
lattice vectors of this supercell were the standard for all slab
thicknesses, because then I could guarantee that the effective k-point
sampling would be the same for all supercells. Since in siesta you
practically don't pay for the vacuum, it's ok to exaggerate and have the
exact same sampling for all cells
After all the slabs had been relaxed, I calculated the surface formation
energy for each of the slabs (equation in LaTeX):
$E^s_n=\frac{E_n-nE_{bulk}}{A}$
where n is the number of layers of the material in the slab, E_{bulk} is
the energy of one layer of the material for the bulk crystal and A is the
surface area of the supercell.
Hope this helps.
Marcos
Vous avez écrit / You have written / Lei ha scritto / Você escreveu... Ali
Jaafar
> Dear all,
> I hope that there is some of you who has studied surfaces or thin
> layers and has an idea on how to calculate the energy of the surface,
> specifically the surface of the Au. I have calculated the energy surface
> of the Au (100), (110) and (111), with 8, 12 and 15 layers and 4 plans
> empty.
>
> I am expecting that surface energy would converge when increasing the
> number of layers but it is not the case in my calculations. The energy
> always increase when adding layers in the case of (100) and (110) and
> always decrease in the case of (111).
> Has anyone any idea of the problem?
>
--
Dr. Marcos Verissimo Alves
Post-Doctoral Fellow
Unité de Physico-Chimie et de Physique des Matériaux (PCPM)
Université Catholique de Louvain
1 Place Croix du Sud, B-1348
Louvain-la-Neuve
Belgique
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