Cd Si volume relaxation

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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 $a=5.4687$ $\mathrm {\AA}$ (volume scan plus Murnaghan EOS using ENMAX=400).

From relaxations using IBRION=2 and ISIF=3 we get $a=5.4684$ $\mathrm {\AA}$ .

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

Download

diamondSivolrel.tgz

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