[Pw_forum] [100] surface relaxation of Al

[Denis Davydov]

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. 
The input script is below. If anyone can spot something suspicious, please let me know. 

I also did a convergence study on Al.pz-vbc.UPF potential for bulk crystal. 
So it pretty much converged in terms of total energy
for ecutwfc, degauss and number of K points i used. 


Kind regards,
Denis

---- pw.x input file ----

&CONTROL
  calculation = "relax",
  prefix = 'Al_lammps'
/
&SYSTEM
  ibrav       = 8,
  celldm(1)   = 60,
  celldm(2)   = 0.125,
  celldm(3)   = 0.125,
  nat         = 20,
  ntyp        = 1,
  ecutwfc     = 35,
  occupations = "smearing",
  smearing    = "marzari-vanderbilt",
  degauss     = 0.005,
/
&ELECTRONS
  conv_thr    = 1e-07,
  mixing_beta = 0.7,
/
&IONS
  bfgs_ndim   = 3,
/
ATOMIC_SPECIES
Al  1.0  Al.pz-vbc.UPF
ATOMIC_POSITIONS bohr
Al   22.5      1.875     1.875    1      1     1
Al   26.25      5.625     1.875    1      1     1
Al   26.25      1.875     5.625    1      1     1
Al   22.5      5.625     5.625    1      1     1
Al   30      1.875     1.875    1      1     1
Al   33.75      5.625     1.875    1      1     1
Al   33.75      1.875     5.625    1      1     1
Al   30      5.625     5.625    1      1     1
Al   37.5      1.875     1.875    1      1     1
Al   41.25      5.625     1.875    1      1     1
Al   41.25      1.875     5.625    1      1     1
Al   37.5      5.625     5.625    1      1     1
Al   45      1.875     1.875    1      1     1
Al   48.75      5.625     1.875    1      1     1
Al   48.75      1.875     5.625    1      1     1
Al   45      5.625     5.625    1      1     1
Al   52.5      1.875     1.875    0      0     0
Al   56.25      5.625     1.875    0      0     0
Al   56.25      1.875     5.625    0      0     0
Al   52.5      5.625     5.625    0      0     0
K_POINTS (automatic)
  4 16 16 1 1 1