[Pw_forum] [100] surface relaxation of Al

[Nicola Marzari]

Hi Denis,

not sure anything looks wrong, a part that your smearing is quite small,
and you might want to test that and the k-points used.

In any case, you could try to leave all atoms free to relax, and study 
interlayer relaxations as a function of thickness, and see how the outer
interlayer relaxations converge to their true value as a function of 
thickness (and progressively the inner relaxations as well).

Last - no need to sampe in the x direction - your system
should not have any band dispersion there (if it has, there is not
enough vacuum).

Hence, 1 16 16 0 1 1 is a very good sampling (even 1 8 8 0 1 1 would
be great, provided you use maybe a more reasonable smearing of .01-.02).


				nicola



On 05/05/2013 12:38, Denis Davydov wrote:
> Dear all,
>
> while learning how QE works, i'm trying to relax ion positions at the [100] surface
> of FCC Aluminum.
>
> Looking at Exercise 4 of day1, QE workshop (2012),
> i made an input file to relax ion position (starting from ideal) while keeping
> 2 layers fixed in the "bulk" (asymmetric slab model).
> I consider 5 unit cells + 3 vacuum in X direction (10 layers) and 1 unit cell in Y/Z.
>
> Results of layers displacements (Bohr) as well as change in interlayer distance relative to
> the perfect distance (7.5 / 2 Bohr ) in % are as follows:
>
> 0 -0.015027115 -0.00602681546667
> 1 0.007573443 -0.0030597528
> 2 0.019047516 0.0012871304
> 3 0.014220777 0.002283688
> 4 0.005656947 0.00307896053333
> 5 -0.005889155 0.00220971653333
> 6 -0.014175592 0.000569633599999
> 7 -0.016311718 -0.004349796
> 8 1.69999978539e-08 -5.33333377462e-10
> 9 1.89999980194e-08
>
> This looks wrong to me. To have some reference I did the same calculation in LAMMPS using EAM potential
> and atom displacements in X are in range of
> -0.190234 to -0.0226957 Angstroms, monotonously decreasing towards the fixed layers, as i would expect it to be.
>
> It seems something is wrong  with DFT calculations (relaxation) i did.
----------------------------------------------------------------------
Prof Nicola Marzari, Chair of Theory and Simulation of Materials, EPFL