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# Cd Si

Volume relaxtion and the calculation of the DOS and bandstructure within cubic diamond (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

• Cubic diamond Si starting lattice constant of 5.5 $\mathrm {\AA}$ .
• Fcc cell.
• 2 atoms in cell.

### INCAR

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


### KPOINTS

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


## Calculation

The calculation of the optimized volume. DOS and bandstructure is similar as in the examples fcc_Si, fcc_Si_DOS and fcc_Si_bandstructure, respectively.

### Volume relaxation

• The bash-script loop.sh runs Si in the cubic diamond (cd) structure at several different lattice constants (5.1-5.7 Å) and collects free energy versus lattice constant into the file SUMMARY.dia:
#! /bin/bash
BIN=/path/to/your/vasp/executable
rm WAVECAR SUMMARY.dia
for i in  5.1 5.2 5.3 5.4 5.5 5.6 5.7 ; do
cat >POSCAR <<!
cubic diamond
$i 0.0 0.5 0.5 0.5 0.0 0.5 0.5 0.5 0.0 2 Direct -0.125 -0.125 -0.125 0.125 0.125 0.125 ! echo "a=$i" ; mpirun -n 2 $BIN E=awk '/F=/ {print$0}' OSZICAR ; echo $i$E  >>SUMMARY.dia
done
cat SUMMARY.dia

• Example output of SUMMARY.dia:
5.2 1 F= -.10528151E+02 E0= -.10528137E+02 d E =-.274709E-04
5.3 1 F= -.10713281E+02 E0= -.10713280E+02 d E =-.218410E-05
5.4 1 F= -.10806685E+02 E0= -.10806685E+02 d E =-.114401E-06
5.5 1 F= -.10823039E+02 E0= -.10823039E+02 d E =-.429842E-08
5.6 1 F= -.10775102E+02 E0= -.10775102E+02 d E =-.204668E-09
5.7 1 F= -.10673578E+02 E0= -.10673578E+02 d E =-.112715E-10
5.8 1 F= -.10528393E+02 E0= -.10528393E+02 d E =-.552513E-11

• To make a quick plot of SUMMARY.dia try:
gnuplot
gnuplot> plot "SUMMARY.dia" using ($1):($4) w lp

• Extracted lattice parameter should be at 5.465 $\mathrm {\AA}$ .

### DOS

• Enter (approximate) volume of 5.5 $\mathrm {\AA}$ into the POSCAR file.
• Change the INCAR according to DOS calculation (or use INCAR.dos):
System = diamond Si
ISTART = 0 ; ICHARG=2
ENCUT  =    240
ISMEAR = -5
LORBIT = 11

• Use p4vasp or run the script dos.sh to calculate the DOS.
• The example DOS should look like this:

### Bandstructure

• Enter (approximate) volume of 5.5 $\mathrm {\AA}$ into the POSCAR file.
• Change the INCAR according to bandstructure calculation (or use INCAR.band):
System = diamond Si
ISTART = 0 ; ICHARG=11
ENCUT  =    240
ISMEAR = 0; SIGMA = 0.1;
LORBIT = 11

• Use p4vasp or run the script band.sh to calculate the bandstructure.
• The example bandstructure should look like this:
• For "fat" bands (or orbital character of bands) use p4vasp:

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).