[gpaw-users] strange behaviour of TB09: vanishing Eg of Si

[Michal Krompiec]

Hello,
I noticed a weird behaviour of TB09. When I do a calculation on Si with
default settings regarding the number of bands, I get a finite Eg as
expected. But when I add more bands, the band gap disappears.
See attached code and DOS plots. What is happening here?? BTW, this seems
specific to TB09 (no such issue with PBE, SCAN, LDA...).

import matplotlib as mpl
mpl.use('Agg')
from matplotlib import pyplot as plt
from pathlib import Path

from ase.build import bulk
from ase.parallel import paropen, world

from gpaw import GPAW, FermiDirac,PW, MarzariVanderbilt
from gpaw.response.df import DielectricFunction

# Ground state calculation
a = 5.431
atoms = bulk('Si', 'diamond', a=a)

calc = GPAW(mode=PW(800),
            kpts={'density': 5.0, 'gamma': True},
            parallel={'band': 1, 'domain': 1},
            xc='TB09',
            occupations=MarzariVanderbilt(0.001))  # use small FD smearing

atoms.set_calculator(calc)
atoms.get_potential_energy()  # get ground state density
# Restart Calculation with fixed density and dense kpoint sampling
e, dos = calc.get_dos(spin=0, npts=2001, width=0.1)
e_f = calc.get_fermi_level()
plt.plot(e - e_f, dos)
plt.axis([-15, 10, None, 4])
plt.ylabel('DOS')
plt.savefig("dos1.png")
calc.set(kpts={'density': 15.0, 'gamma': True},  # dense kpoint sampling
         fixdensity=True)
atoms.get_potential_energy()
calc.diagonalize_full_hamiltonian(nbands=70)  # diagonalize Hamiltonian
calc.write('si_large.gpw', 'all')  # write wavefunctions
e, dos = calc.get_dos(spin=0, npts=2001, width=0.1)
e_f = calc.get_fermi_level()
plt.plot(e - e_f, dos)
plt.axis([-15, 10, None, 4])
plt.ylabel('DOS')
plt.savefig("dos2.png")

Best,
Michal Krompiec
[image: dos2.png]
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