Fcc Si

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Task

Lattice constant optimization for fcc Si.

Input

POSCAR

fcc Si:
 3.9
 0.5 0.5 0.0
 0.0 0.5 0.5
 0.5 0.0 0.5
   1
cartesian
0 0 0

• Fcc Si lattice constant of 3.9 $\AA$.
• 1 atom per unit cell.

INCAR

System = fcc Si
ISTART = 0 ; ICHARG = 2
ENCUT = 240
ISMEAR = 0; SIGMA = 0.1

• Initial charge density form overlapping atoms.
• Energy cutoff of 240 eV from POTCAR file.

KPOINTS

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

• Equally spaced k mesh.
• Odd number of k points in each direction results in a $\Gamma$ centered mesh.
• 56 k points in IBZ.

Calculation

• Calculate energy for different lattice parameters.
• Fit to some equation of states to obtain the equilibrium volume.

• The bash-script loop.sh runs fcc Si at several different lattice constants (3.5-4.3 Å) and collects free energy versus lattice constant into the file SUMMARY.fcc

#! /bin/bash
BIN=/path/to/your/vasp/executable
rm WAVECAR SUMMARY.fcc
for i in  3.5 3.6 3.7 3.8 3.9 4.0 4.1 4.2 4.3 ; do
cat >POSCAR <<!
fcc:
   $i
 0.5 0.5 0.0
 0.0 0.5 0.5
 0.5 0.0 0.5
   1
cartesian
0 0 0
!
echo "a= $i" ; mpirun -np 2 $BIN
E=`awk '/F=/ {print $0}' OSZICAR` ; echo $i $E  >>SUMMARY.fcc
done
cat SUMMARY.fcc

Mind: You will have to set the correct path to your VASP executable (i.e., BIN), and invoke VASP with the correct command (e.g., in the above: mpirun -np 2).

• To make a quick plot of SUMMARY.fcc try:

gnuplot
gnuplot> plot "SUMMARY.fcc" using ($1):($4) w lp

Download

fccSi.tgz

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