## Ni100 surface

Based on the three VASP wiki examples in the links 1, 2 and 3

Task: Perform a relaxation of the first two layers of a Ni (100) surface, thereafter calculate its DOS and bandstructure.

First, copy the example folder which contains some of the VASP input files

``````cp -r /software/sse/manual/vasp/training/ws2020/Ni100_surf .
cd Ni100_surf``````

and copy the latest POTCAR file for Ni

``cp /software/sse/manual/vasp/POTCARs/PBE/2015-09-21/Ni/POTCAR .``

### Input files

POSCAR

``````fcc (100) surface
3.53
.50000   .50000   .00000
-.50000   .50000   .00000
.00000   .00000  5.00000
Ni
5
Selective Dynamics
Cartesian
.00000   .00000   .00000 F F F
.00000   .50000   .50000 F F F
.00000   .00000  1.00000 F F F
.00000   .50000  1.50000 T T T
.00000   .00000  2.00000 T T T``````
• Ni lattice constant a = 3.53Å
• 1 atom per layer
• 5 layers in total
• Note "S" or "Selective dynamics" chosen in the line under number of atoms

INCAR

``````  ISTART = 0; ICHARG = 2

general:
SYSTEM = clean Ni(100) surface
ENCUT = 270
ISMEAR = 2 ; SIGMA = 0.2
ALGO = Fast
EDIFF = 1E-6

spin:
ISPIN=2
MAGMOM = 5*1

dynamic:
NSW = 100
POTIM = 0.8
IBRION = 1``````
• ISTART=0, static calculation (default)
• ICHARG=2, initial charge-density from overlapping atoms (default if ISTART=0)
• ENCUT=270, default energy cutoff 270 eV
• ISMEAR=2, Methfessel-Paxton smearing used for metal
• SIGMA=0.2, default smearing
• ALGO=Fast, (default is Normal) first steps according to Davidson, thereafter RMM-DIIS, algorithms for electron minimization
• EDIFF=1E-6,
• ISPIN=2, gives a spin-polarized calculation
• MAGMOM=5*1, for the 5 atoms in POSCAR an initial magnetic moment of 1 Bohr magnetons
• NSW=100,
• POTIM=0.8,
• IBRION=1, ionic relaxation (RMM-DIIS)

KPOINTS

``````k-points
0
Monkhorst-Pack
9 9 1
0 0 0``````
• Equally spaced k-mesh
• Odd Monkhorst-Pack k-mesh > Gamma centered
• Note, only one k-point in the z-direction for the surface

### 1. Surface relaxation

First, note how selective dynamics (S) works in the POSCAR file

``````Selective Dynamics
Cartesian
.00000   .00000   .00000 F F F
.00000   .50000   .50000 F F F
.00000   .00000  1.00000 F F F
.00000   .50000  1.50000 T T T
.00000   .00000  2.00000 T T T``````

So, for relaxation `F=False` and `T=True`, so one sees that the two atoms at z=1.5 and 2.0 are able to move in all directions, while the three lower atoms (layers) are fixed.

• For surface relaxations it's typical to fix the bulk-like atoms in the bottom and let the ones close to the surface be able to relax
• Alternatively, a symmetric supercell can be constructed with inner bulk-like layers

Run the calculation with

``sbatch run.sh``

and observe the relaxation e.g. with

``cat OSZICAR``

After the calculation is finished, compare the forces in OUTCAR between the first and the last step.

First step:

`````` POSITION                                       TOTAL-FORCE (eV/Angst)
-----------------------------------------------------------------------------------
0.00000      0.00000      0.00000         0.000000      0.000000      0.419201
0.00000      1.76500      1.76500         0.000000      0.000000     -0.401608
0.00000      0.00000      3.53000         0.000000      0.000000     -0.002589
0.00000      1.76500      5.29500         0.000000      0.000000      0.392849
0.00000      0.00000      7.06000        -0.000000     -0.000000     -0.407852
-----------------------------------------------------------------------------------
total drift:                               -0.000000     -0.000000     -0.008050``````

Last step:

`````` POSITION                                       TOTAL-FORCE (eV/Angst)
-----------------------------------------------------------------------------------
0.00000      0.00000      0.00000        -0.000000      0.000000      0.425910
0.00000      1.76500      1.76500         0.000000     -0.000000     -0.402950
0.00000      0.00000      3.53000        -0.000000      0.000000     -0.023012
0.00000      1.76500      5.29871        -0.000000      0.000000     -0.001056
-0.00000     -0.00000      6.98070        -0.000000     -0.000000      0.001107
-----------------------------------------------------------------------------------
total drift:                                0.000000     -0.000000     -0.019988    ``````

Check the obtained relaxed structure in `CONTCAR`:

``````fcc (100) surface
3.53000000000000
0.5000000000000000    0.5000000000000000    0.0000000000000000
-0.5000000000000000    0.5000000000000000    0.0000000000000000
0.0000000000000000    0.0000000000000000    5.0000000000000000
Ni
5
Selective dynamics
Direct
0.0000000000000000  0.0000000000000000  0.0000000000000000   F   F   F
0.5000000000000000  0.5000000000000000  0.1000000000000014   F   F   F
0.0000000000000000  0.0000000000000000  0.2000000000000028   F   F   F
0.5000000000000000  0.5000000000000000  0.3002099841022341   T   T   T
-0.0000000000000000  0.0000000000000000  0.3955072458335201   T   T   T``````
• Check the convergence using p4vasp
• Compare surface energies following the example in the VASP wiki
• What is the inward relaxation of the surface layers (compare VASP wiki)
• Visualize the relaxation of the structure using p4vasp (follow VASP wiki figures)

### 2. Surface DOS

Now, in a new folder "dos", calculate the local DOS for the relaxed Ni(100) surface using CONTCAR from the previous step

``````mkdir dos
cp CONTCAR dos/POSCAR
cp INCAR.dos dos/INCAR
cp POTCAR KPOINTS run.sh dos
cd dos``````

Also note that we use a new INCAR (from INCAR.dos) which looks like

``````general:
SYSTEM = clean (100) Ni surface
ENMAX = 270
ISMEAR =   -5
ALGO = Normal

spin:
ISPIN = 2
MAGMOM = 5*1

LORBIT = 11  # lm and site decomposed DOS inside PAW spheres``````
• ISMEAR=5, the tetrahedron method with Blöchl corrections, suitable for DOS
• ALGO=Normal, the default electron minimization algorithm
• LORBIT=11, lm and site decomposed DOS

Run the calculation with

``sbatch run.sh``

After it finishes, at the end of OUTCAR, check the information on local charge and magnetization

`````` total charge

# of ion       s       p       d       tot
------------------------------------------
1        0.464   0.328   8.294   9.086
2        0.488   0.483   8.309   9.280
3        0.491   0.485   8.317   9.294
4        0.498   0.505   8.334   9.338
5        0.477   0.350   8.332   9.158
--------------------------------------------------
tot          2.418   2.151  41.586  46.156

magnetization (x)

# of ion       s       p       d       tot
------------------------------------------
1       -0.003  -0.021   0.751   0.727
2       -0.008  -0.026   0.645   0.611
3       -0.008  -0.026   0.636   0.602
4       -0.008  -0.026   0.630   0.596
5       -0.004  -0.021   0.725   0.700
--------------------------------------------------
tot         -0.032  -0.120   3.388   3.236``````
• By setting LORBIT=1 and changing RWIGS, it's possible to control the total number of electrons within the sphere
• What can be said about the magnetic moments for the different layers?
• Test to plot DOS using p4vasp, see figures at the end of the VASP wiki example

### 3. Surface bandstructure

Now, calculate the corresponding bandstructure for the relaxed Ni(100) surface structure. Go to the main folder "Ni100_surf" and there create the folder "band" and copy the needed files

``````mkdir band
cp CONTCAR band/POSCAR
cp INCAR.band band/INCAR
cp KPOINTS.band band/KPOINTS
cp POTCAR run.sh band
cd band``````

Also note that we use a new INCAR (from INCAR.band) which looks like

``````  ICHARG = 11
general:
SYSTEM = clean (100) nickel surface
ENMAX  = 270
ISMEAR = 2 ; SIGMA = 0.2
ALGO = Normal

spin:
ISPIN = 2
MAGMOM = 5*1

LORBIT = 11``````
• ICHARG=11, for a non-self consistent run using previously obtained CHGCAR

Copy CHGCAR

``cp ../dos/CHGCAR .``

This time KPOINTS for the bandstructure looks like

``````kpoints for band-structure G-X-M-G
13
reziprok
.00000   .00000   .00000    1
.12500   .00000   .00000    1
.25000   .00000   .00000    1
.37500   .00000   .00000    1
.50000   .00000   .00000    1

.50000   .12500   .00000    1
.50000   .25000   .00000    1
.50000   .37500   .00000    1
.50000   .50000   .00000    1

.37500   .37500   .00000    1
.25000   .25000   .00000    1
.12500   .12500   .00000    1
.00000   .00000   .00000    1  ``````
• 13 k-points along the line Gamma - X - M - Gamma
• Reciprocal coordinates
• Each point has weight 1

Submit the job

``sbatch run.sh``

and wait for it to finish. Investigate the bandstructure using p4vasp, compare with the VASP wiki example.

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