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

[Michal Krompiec]

Thanks for looking into this!
Best,
Michal

On Fri, Jun 19, 2020 at 8:53 AM Jens Jørgen Mortensen <jjmo at dtu.dk> wrote:

> Den 17.06.2020 kl. 12.03 skrev Michal Krompiec via gpaw-users:
> > 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...).
>
> It looks like MGGA's and TB09 (which is sort of an MGGA) don't work with
> fixdensity=True.  The kinetic energy density gets initialized to zero
> and then never updated.  For now, you should avoid fixdensity=True for
> those functionals.  SCAN is an MGGA, did it really work for you?
>
> https://gitlab.com/gpaw/gpaw/-/issues/270
>
> Jens Jørgen
>
> > 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
> > dos2.png
> >
> > _______________________________________________
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> >
>
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