Nanosurf Lab

to calculate the band structure for given lattice parameter we need to achieve SCF solution. Therefore we run a standard bulk calculation as described in previous chapter. Let's calculate the band structure for the lattice parameter alat = 5.5 Å, than the fireball.in file contains:

&OPTION
basisfile = Si.bas
lvsfile = Si.lvs
kptpreference = Si.kpts
nstepf = 1
rescal = 5.5
&END

where Si.bas, Si.lvs and Si.kpts files are identical as those used in previous chapter. Once we obtain the SCF solution, we determine the Fermi level from output file

mac135> grep "Fermi Level" $output_file | tail -1
 Fermi Level =   -4.33045968490834 

In next step, we run the FIREBALL code again, but now with fixed charges (ifixcharge = 1) and with a new set of k-points in desired high-symmetry directions in the first Brillouin zone. Remember we need having CHARGES file in a working directory for a restart. In this particular case,we have chosen a direction L-Γ-X-Γ stored in a lgxg.kpts file. In addition, we have to write out a list of eigenvalues at each k-point switching on iwrteigen variable. Our input file fireball.in has following form now:

&OPTION
basisfile = Si.bas
lvsfile = Si.lvs
kptpreference = lgxg.kpts
nstepf = 1
ifixcharge = 1
rescal = 5.5
&END
&OUTPUT
iwrteigen = 1
&END

After the run, we obtain a file ek.dat appears in a working directory. This file contains at each line set of eigenvalues for given k-points ordered in ascending form. The Fermi level can Now we have all information to plot the band structure of the Si bulk.

mac135> gnuplot
gnuplot> set xrange [0:300]
gnuplot> set yrange [-17:0]
gnuplot> set xlabel "k-points"
gnuplot> set ylabel "Energy [eV]"
gnuplot> set nokey
gnuplot> set multiplot
multiplot> plot "ek.dat" using 1:2 with lines
multiplot> plot "ek.dat" using 1:3 with lines
multiplot> plot "ek.dat" using 1:4 with lines
multiplot> plot "ek.dat" using 1:5 with lines
multiplot> plot "ek.dat" using 1:6 with lines
multiplot> plot "ek.dat" using 1:7 with lines