Cd Si volume relaxation

From VASP Wiki

Task

Relaxation of the internal coordinates, volume and cell shape in cd Si.

Input

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 -0.125
  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 conjugate-gradient algorithm.
  • ISIF=3 change of internal parameter, shape and volume simultaneously.

KPOINTS

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

Calculation

  • To determine the equilibrium volume we can:
    • Fit the energz over a certain volume range to an equation of state (see cd_Si).
    • Alternatively we relax the structure with VASP "on the fly" (IBRION=2 and ISIF=3)
  • From equation of states we determine lattice parameter of Å (volume scan plus Murnaghan EOS using ENMAX=400).
  • From relaxations using IBRION=2 and ISIF=3 we get Å.
  • Difference can be due to pulay stress (especially when the relaxation starts far away from equilibrium):
-------------------------------------------------------------------------------------
Total       0.00155     0.00155     0.00155    -0.00000     -0.00000      0.00000
in kB       0.06056     0.06056     0.06056    -0.00000     -0.00000      0.00000
external pressure =        0.06 kB  Pullay stress =          0.00 kB
   
   
VOLUME and BASIS-vectors are now :
-----------------------------------------------------------------------------
 energy-cutoff :      400.00
 volume of cell :      40.88
     direct lattice vectors                 reciprocal lattice vectors
    0.000000000  2.734185321  2.734185321    -0.182869828  0.182869828  0.182869828
    2.734185321  0.000000000  2.734185321     0.182869828 -0.182869828  0.182869828
    2.734185321  2.734185321  0.000000000     0.182869828  0.182869828 -0.182869828


  • To remedy this increase the plane wave cutoff by at least 30% (here we used ENMAX=400 instead of 240) and use a small EDIFF.

Summary

  • Calculation of the equilibrium volume:
    • FIt the energy over a certain volume range to an equation of state.
    • When internal degrees of freedom exist (e.g. c/a), the structure must be optimized. Use a conjugate-gradient algorithm (IBRION=2) and at each volume do e.g. 10 ionic steps (NSW=10) and allow change of internal parameters and shape (ISIF=4).
  • Simpler but less reliable: relaxing all degrees of freedom including volume.
    • To relax all degrees of freedom use ISIF=3 (internal coordinates, shape and volume).
    • Mind pulay stress problem. Increase plane wave cutoff by 25-30% when the volume is allowed to change.

Download

diamondSivolrel.tgz

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