## cubic diamond Si volume relaxation

Based on the VASP wiki example in this link

Task: Relaxation of the internal coordinates, volume and cell shape in cubic diamond Si. Also see the discussion on energy vs volume, volume relaxations and Pulay stress

There are several ways in which the equilibrium volume can be determined

• By fitting the total energies over a volume range e.g. using an equation of state method
• Relaxing the structure with VASP "on the fly"

The first approach was done in the previous example cd Si. Here, the second approach will be used. While it's fast, one needs to be careful regarding the accuracy and consider the Pulay stress.

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

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

and copy the latest POTCAR file for Si:

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

### Input files

POSCAR

``````cubic diamond Si
5.5
0.0    0.5     0.5
0.5    0.0     0.5
0.5    0.5     0.0
Si
2
Direct
-0.125 -0.125 -0.125
0.125  0.125  0.125``````

INCAR

``````System = diamond Si

ISMEAR    = 0
SIGMA     = 0.1
ENMAX     = 240
IBRION    = 2
ISIF      = 3
NSW       = 15
EDIFF     = 0.1E-04
EDIFFG    = -0.01``````
• IBRION=2, for using the conjugate-gradient algorithm
• ISIF=3, for change of internal parameters, shape and volume at the same time
• NSW=15, max number of ionic steps
• EDIFF=0.1E-04, the default stop value for difference in total energy between steps
• EDIFFG=-0.01, if negative value, stop criterium for ionic relaxation if forces are smaller than |EDIFFG|

KPOINTS

``````k-points
0
Monkhorst Pack
11 11 11
0  0  0``````

### Calculation

Start the volume relaxation calculation by

``sbatch run.sh``

Monitor how the structural relaxation progresses with

``cat OSZICAR``

Check the relaxed final structure

``cat CONTCAR``
• Compare with the EOS lattice parameter obtained in the previous example and from the VASP wiki a= 5.4687 Å

• Note that one doesn't use the total energy or DOS from a structural relaxation. For that, CONTCAR should be copied to POSCAR for a separate caclulation with no structural relaxation.

Check the convergence of the total energy and forces using p4vasp

``p4v &``

by selecting Convergence > Energy or > Forces.

• Repeat the volume relaxation calculation in a new folder, but this time start from the relaxed structure `CONTCAR`, by copying it to POSCAR. Does the structure change, if so, how much?

• Repeat the volume relaxation calculation from scratch with higher accuracy, e.g. set ENCUT to 1.3 x ENMAX (grep ENMAX POTCAR) and PREC=Accurate in INCAR. Also, set a smaller value for EDIFF. Any difference in the lattice parameter a?

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System status

#### Everything OK!

No reported problems