[Pw_forum] Problem With Total-Energy Convergence vs. ecutwfc
Vic Bermudez
victor.bermudez at nrl.navy.mil
Mon Nov 21 21:32:09 CET 2011
Hello,
I seem to be having some serious trouble getting total energies to converge as I vary
"ecutwfc" and "ecutrho". This is for a slab-model calculation, using PBE and USPP's, for
an HF molecule reacting with the surface of Si3N4. The total energy results I get in
representative calculations are:
ecutwfc = 40 ecutrho = 400 Etot = -1504.8173322951
ecutwfc = 50 ecutrho = 500 Etot = -1505.03408093
ecutwfc = 60 ecutrho =720 Etot = -1505.12486679
>From what I'm reading in various posts in the Users' Forum, in tutorials available on the
web and in the Users' Guide, these values of "ecutwfc" and "ecutrho" are greatly in excess
of what should be needed to achieve convergence.
This raises the question of how to define convergence. One Users' Forum post said that a
convenient criterion is 1 mryd/atom. Since my slab unit cell contains 91 atoms, the
difference between 40/400 and 50/500 is 2.4 mryd/atom. Is it total energy or total energy
per atom that I should be looking at ?
What I'm actually interested in is reaction energies (or adsorption energies). The
reaction energies I get for 40/400 and 60/720 are -0.316 and -0.317 eV respectively. In
other words, they're identical, because the energy differences between 40/400 and 60/720
for the bare slab and for the slab+adsorbate cancel. I'm not sure whether I should be
relieved by this or if I should be wary of a hidden problem.
As another comment, results for 6x6x1 and 12x12x1 Monkhorst-Pack grids are virtually
identical (with ecutwfc=40 and ecutrho=400 in both cases). Hence, k-point spacing doesn't
appear to be an issue.
I'm inserting below a typical input file here:
&CONTROL
calculation='scf',
title='repeat energy calculation like #50400 but bigger ecutwfc and ecutrho',
pseudo_dir='/lustre/cmf/scratch/b/bermudez/QE_PP/',
verbosity='default'
/
&SYSTEM
ibrav=8,
a=5.860, b=7.673, c=53.00,
nat=91,
ntyp=5,
ecutwfc=50.0,
ecutrho=500.0,
occupations='fixed',
nosym_evc=.TRUE.
/
&ELECTRONS
electron_maxstep=100,
conv_thr=1.0D-9,
mixing_beta=0.7D0,
mixing_ndim=8,
mixing_mode='plain'
/
ATOMIC_SPECIES
Si 28.0855 Si_pbe-n-van_UPF
N 14.0067 N_pbe-van_ak_UPF
O 15.9994 O_pbe-van_ak_UPF
F 18.9984032 F_pbe-n-van_UPF
H 1.00794 H_pbe-van_ak_UPF
ATOMIC_POSITIONS angstrom
F -0.828943122 -3.842610696 10.898762889
H 2.956336746 -3.913628193 11.481740207
O 2.130945237 -3.743406775 10.990758649
Si -0.740055896 -3.727473495 9.278104350
Si 2.197736003 -3.719125735 9.357412872
O 0.733635389 -0.092404715 8.709367118
O -2.198390199 -0.092106250 8.703606145
N 0.727690134 -2.912055280 8.831492617
N -2.199510369 -2.913961388 8.797211298
N -0.730334009 2.291265533 8.708545486
N 2.200026883 2.299996238 8.715832903
Si 0.740325891 1.486240963 8.197839727
Si -2.201425602 1.487223849 8.194133645
Si 0.731520021 -1.484390893 7.805743483
Si -2.199544503 -1.481807201 7.794749488
N -0.730477119 -1.509950402 6.851115290
N 2.192766376 -1.509070971 6.850785646
N 0.734572342 1.428876787 6.443023829
N -2.196689576 1.428710465 6.440830516
Si -0.730671582 1.383061493 5.485559504
Si 2.198231920 1.383467537 5.484097670
Si -0.731571258 -1.634722031 5.102794951
Si 2.196186337 -1.634652146 5.102809652
N 0.732378551 -2.470977191 4.637802788
N -2.197580366 -2.470463034 4.640966741
N -0.731691807 2.788232427 4.443687098
N 2.197979174 2.788401786 4.442656379
N -0.731484010 -0.019627141 4.441442429
N 2.198010976 -0.019733235 4.440785365
Si 0.732965820 3.599969198 3.935485972
Si -2.196881366 3.600616053 3.936927345
Si -0.732092418 0.194969145 2.706451385
Si 2.197948022 0.194942344 2.705997893
N 0.732838523 3.815828133 2.206216987
N -2.196890255 3.816167783 2.207269314
N 0.732842940 1.010463084 2.200606165
N -2.197021884 1.010498140 2.200720312
N -0.732301333 -1.402362859 1.988160033
N 2.197752904 -1.402429612 1.987932157
Si 0.732683992 -2.247692417 1.532621948
Si -2.197244178 -2.247708827 1.532715316
Si 0.732816753 2.419773565 1.160763975
Si -2.197113870 2.420131824 1.161325553
N -0.732304509 2.385786603 0.203241957
N 2.197969280 2.385645498 0.203316922
N 0.732493393 -2.433993290 -0.207756507
N -2.197450470 -2.434258734 -0.207663162
Si -0.732495688 -2.466691676 -1.166181979
Si 2.197512928 -2.466630603 -1.166105788
Si -0.732435027 2.205350148 -1.537697562
Si 2.197675396 2.205337618 -1.537670714
N 0.732607176 1.361450666 -1.991660707
N -2.197386115 1.361482103 -1.991967809
N -0.732555455 -1.052728897 -2.197325278
N 2.197485775 -1.052686623 -2.197266201
N -0.732575216 3.812491336 -2.215973946
N 2.197423357 3.812523616 -2.215877477
Si 0.732455781 -0.237903510 -2.702429787
Si -2.197536589 -0.237927098 -2.702546727
Si -0.732782356 -3.644946429 -3.947788648
Si 2.197212071 -3.644957994 -3.947708493
N 0.732186804 -0.022261163 -4.433181470
N -2.197780158 -0.022299546 -4.433277495
N 0.732217627 -2.830367771 -4.450667073
N -2.197813068 -2.830395474 -4.450700148
N -0.732746654 2.428899633 -4.661913346
N 2.197243946 2.428854425 -4.661795907
Si 0.732252914 1.582997684 -5.112922128
Si -2.197749414 1.582996773 -5.112924046
Si 0.732131635 -1.416752401 -5.483156140
Si -2.197874184 -1.416795000 -5.483202539
N -0.732884377 -1.446757009 -6.437077490
N 2.197101491 -1.446724700 -6.437084206
N 0.732395894 1.404310755 -6.857972381
N -2.197609347 1.404357298 -6.857976982
Si -0.732500000 1.391882200 -7.807890185 0 0 0
Si 2.197500000 1.391882200 -7.807890185 0 0 0
Si -0.732500000 -1.607109850 -8.180675410 0 0 0
Si 2.197500000 -1.607109850 -8.180675410 0 0 0
N 0.732500000 -2.451907150 -8.633865291 0 0 0
N -2.197500000 -2.451907150 -8.633865291 0 0 0
N -0.732500000 2.806399750 -8.838531689 0 0 0
N 2.197500000 2.806399750 -8.838531689 0 0 0
O -0.732500000 0.000000000 -8.860017231 0 0 0
O 2.197500000 0.000000000 -8.860017231 0 0 0
Si 0.732500000 3.621272350 -9.343552671 0 0 0
Si -2.197500000 3.621272350 -9.343552671 0 0 0
O 0.823729487 3.733135449 -10.991705035 0 0 0
O -2.106270809 3.733135519 -10.991705242 0 0 0
H -0.038874881 3.812689877 -11.453270153 0 0 0
H -2.968874621 3.812689805 -11.453270009 0 0 0
K_POINTS automatic
6 6 1 0 0 0
As always, thanks in advance for whatever help and advice you can provide.
Best Wishes,
Vic Bermudez
************
Victor M. Bermudez
Code 6876
U.S. Naval Research Laboratory
4555 Overlook Ave., S.W.
Washington, DC 20375-5347
Phone: 202-767-6728
FAX: 202-767-1165
E-mail: victor.bermudez at nrl.navy.mil
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