How to do an atomic positions optimization (Relaxation)

We will take the example of Silicon in the cubic diamond structure.

 

POSCAR

cubic diamond
   5.5
 0.0    0.5     0.5
 0.5    0.0     0.5
 0.5    0.5     0.0
  2
Direct
 -0.125 -0.125 -0.130
  0.125  0.125  0.130  

• Break of symmetry in standard diamond structure: change z position from 0.125 to 0.130.

 

INCAR

general:
 System = diamond Si
 START = 0 ; ICHARG=2
 ENCUT  =    240
 ISMEAR = 0; SIGMA = 0.1;
 NSW    = 10; IBRION =  2
 ISIF   =  2

 EDIFFG = -0.0001

• 10 relaxation steps (NSW=10).
• Conjugate-gradient algorithm (IBRION=2).
• Relaxation only of internal parameters (ISIF=2).

 

KPOINTS

K-Points
 0
Monkhorst Pack
 11 11 11
 0  0  0

 POTCAR

Download the pseudopotential file from this link diamondSirel.tgz

Calculation

/Ni_dos> vasp_std

 

• Example output after 10 relaxation steps:
  

 POSITION                                       TOTAL-FORCE (eV/Angst)
 -----------------------------------------------------------------------------------
      4.81253      4.81253      4.81250        -0.000723     -0.000723     -0.000012
      0.68747      0.68747      0.68750         0.000723      0.000723      0.000012
 -----------------------------------------------------------------------------------
    total drift:                                0.000000      0.000000      0.000000

 

Files to watch during relaxations:

• • stdout (terminal): each electronic step is written to the terminal:
  • OSZICAR: a copy of the terminal output.
  • OUTCAR: more detailed information on every electronic and ionic step.

• Other important files:
  • CONTCAR: holds the structure of the last ionic step and at the end the structural result (also very important for restarting a relaxation).
  • STOPCAR: stops a relaxation.

 

The content of the CONTCAR file

cubic diamond                           
   5.50000000000000     
     0.0000000000000000    0.5000000000000000    0.5000000000000000
     0.5000000000000000    0.0000000000000000    0.5000000000000000
     0.5000000000000000    0.5000000000000000    0.0000000000000000
   Si
     2
Direct
  0.8750001520105571  0.8750001520105571  0.8750103324243388
  0.1249998479894427  0.1249998479894427  0.1249896675756612
 
  0.00000000E+00  0.00000000E+00  0.00000000E+00
  0.00000000E+00  0.00000000E+00  0.00000000E+00