# [Pw_forum] Problem With Total-Energy Convergence vs. ecutwfc

Nicola Marzari nicola.marzari at epfl.ch
Mon Nov 21 22:52:31 CET 2011

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Dear Vic,

as you correctly point out, differences in total energy are
very well converged in your calculations. This is all what matters.

Absolute total energies are not yet converged, but those are not
relevant or useful - if you think about, what you'll need will always
be differences in energies between different calculations, and those
are the ones you need to converge (as indeed you do).

nicola

On 11/21/11 9:32 PM, Vic Bermudez wrote:
> 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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--

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Prof Nicola Marzari, Chair of Theory and Simulation of Materials, EPFL

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