[QE-users] electronic structure of (001) Nb

[Roozbeh Anvari]

Dear experts,

I need to get the optimal number of layers of (110) niobium that is a good
representative of the behavior of the material, but I am facing the
following problems:

1. I Keep getting the following warning when relaxing the bulk material:
"SCF correction compared to forces is large: reduce conv_thr to get better
values"

I tried reducing conv_thr, increasing ecutwfc and/or K-mesh, and
occupations, but no success yet ( the set up I am using for this
calculation is given below)

2. the surface energy I get from the (110) slab I cleave from this bulk
structure is

significantly increasing with increasing number of layers :

   [ 3L : 0.13814157,  5L :0.14193375,  7L :0.14578974,  9L : 0.15090349,
11L : 0.1554551 , 13L : 0.16026892] // [eV/\AA^2]

 I am wondering whether I am having a problem with the bulk or my surface
calculations or even the PP I am using ( I need to keep using NCPP as I am
going to look into phonon bands),

Thank you for your help,

Roozbeh

PDF, Physics,

Queen's University , Canada


// input file for bulk :

&CONTROL calculation= 'vc-relax', title='ml_gr', prefix='ml_gr',
verbosity='high', restart_mode= 'from_scratch', nstep=200, iprint=1,
tprnfor=.true., outdir='./tmp', disk_io='default',
pseudo_dir='../pp_sg15_oncv/', ! vanderbilt NC PBE, forc_conv_thr=1.0d-6, /

&SYSTEM ibrav=0, nat = 2 ,
ntyp = 1, ecutwfc = 55.0 ,
ecutrho = 440.0, nosym=.true., occupations='smearing', smearing='cold',
degauss=0.02,

vdw_corr='grimme-d3', input_dft='PBE',

/ &ELECTRONS startingwfc='random', diagonalization='cg', conv_thr =
1.0e-12, mixing_beta = 0.2, electron_maxstep=200,

/ &IONS ion_dynamics='bfgs', upscale=20.0, /

&CELL press_conv_thr = 0.5D0, press = 0.D0, cell_dynamics = 'bfgs',
cell_dofree = 'all', cell_factor = 1.5D0, /

ATOMIC_SPECIES Nb 92.906 Nb_ONCV_PBE-1.2.upf ! [Kr] 4d4 5s1,

K_POINTS automatic 12 12 12 0 0 0

CELL_PARAMETERS (angstrom) 3.3205199242 0.0000000000 0.0000000000
0.0000000000 3.3205199242 0.0000000000 0.0000000000 0.0000000000
3.3205199242

ATOMIC_POSITIONS (crystal) Nb 0.000000000 0.000000000 0.000000000 Nb
0.500000000 0.500000000 0.500000000
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