[QE-users] Effect of nbnd on vc-relax

[Jing Lian Ng]

Hello all,

I am trying to do a vc-relax calculation on y-Fe2O3 with 160 atoms.
Currently I am facing convergence issue with the input script below. With
nbnd = 800, the calculation usually finishes in ~ 1 hr on a supercomputer,
but fails to reach convergence even on the first BFGS step. When I removed
the nbnd parameter, QE automatically set the number of Kohn-Sham states to
960 and the calculation was able to proceed. However, the relaxation
calculation is taking more than 72 hours and hasn't reached completion (It
is still running now, on 15th BFGS cycle). Is this a normal timescale for a
system with >100 atoms? And how should I determine an appropriate value of
nbnd before starting any calculation?

############ Input Script ####################
&SYSTEM
   ibrav           = 14
   A               = 8.48998
   B               = 8.48998
   C               = 25.46994
   cosAB           = 0
   cosAC           = 0
   cosBC           = 0
   nat             = 160
   ntyp            = 3
   ecutwfc         = 65
   ecutrho         = 780
   nbnd            = 800
   occupations     = 'smearing'
   degauss         = 0.001
   smearing        = 'mv'
   nspin           = 2
   starting_magnetization(1) = 0.5
   starting_magnetization(2) = -0.5
   starting_magnetization(3) = 0
 /
 &ELECTRONS
   conv_thr        = 0.00001
   mixing_mode     = 'plain'
   mixing_beta     = 0.3
   electron_maxstep = 200
 /
 &IONS
 /
 &CELL
   !cell_dofree     = 'volume'
 /
ATOMIC_SPECIES
  FeO   55.84500  Fe.pbesol-spn-kjpaw_psl.0.2.1.UPF
  FeT   55.84500  Fe.pbesol-spn-kjpaw_psl.0.2.1.UPF
    O   15.99900  O.pbesol-n-kjpaw_psl.0.1.UPF
##########################################################

Thanks,
Jing Lian Ng
Graduate student at CHE, University of Texas at Austin
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