[QE-users] oscillating energy value during optimization

Nicola Marzari nicola.marzari at epfl.ch
Mon Jul 20 16:29:32 CEST 2020



Hi Coralie,


it's not easy to converge "difficult" systems with iterative approaches, 
so it's also a matter of expertise and experience, but here are a few 
pointers:

1) decreasing degauss makes things worse, due to level crossing 
instabilities. It should be around 0.02 Ry, or even larger (but not more 
than 0.03 Ry).

2) I would suggest to use the SSSP pseudopotentials - we had to work on 
N quite a bit. Use the suggested cutoffs for the "efficiency" version. 
If in doubt, run everything on the materials cloud qe input generator.

3) is the number of bands correct? you want to have enough bands to 
cover also states that are starting to be quite empty, but you do not 
want to have a much larger number. how many electrons do you have in 
your calculation?  you want something like 10% or max 20% more nbnd than 
the number of electrons, but I would just let the code decide this for 
you, and then check that the highest states (print verbose) are almost 
empty, but not a lot that are completely empty. If the highest one has 
an occupation that is below 0.01 or 0.005 you are fine.

4) your convergence threshold seems also loose - what does the qe input 
generator tells you? use that (with the same pseudos that come with that 
- the convergence threshold depends on the number of electrons in your 
system). if your calculations are not well converged, the forces are not 
very accurate, so the atoms are moved in wrong directions during the 
relaxation, and thus the total energy from one ionic step to the next 
will not go down.

5) can you try a simpler system first?

6) the fact that you put cosab = 6.12323e-17 rather than zero gets me 
worried - there is no reason to have a number different from zero. Of 
course 10e-17 is identical to zero, but it belies some carelessness.

				nicola


On 20/07/2020 16:05, Coralie Khabbaz wrote:
> Hello,
> 
> My structure is a WN slab with molecules adsorbing on its surface. I 
> always have problems with convergence: everytime I try to relax the 
> structure, multiple scf steps converge, but then the energy value starts 
> oscillating and it doesn't converge anymore. My structure gets stuck 
> every time, even though I visualized it after every scf step and the 
> structure seems fine. This is the input file I am using. I also tried 
> decreasing the degauss to 1e-2, increasing ecutrho and ecutwfc by a 
> little bit, decreasing mixing_beta to 6e-2 and also tried changing the 
> mixing_mode to TF, but nothing worked! Does anyone have advice? Thank you!
> 
> &CONTROL
>      calculation   = "relax"
>      forc_conv_thr =  1.00000e-03
>      max_seconds   =  4.32000e+05
>      nstep         = 200
>      pseudo_dir    = 
> "/home/coralee/projects/def-jkopysci/coralee/.pseudopot"
> /
> 
> &SYSTEM
>      a                         =  8.32716e+00
>      angle1(1)                 =  0.00000e+00
>      angle2(1)                 =  0.00000e+00
>      b                         =  8.98689e+00
>      c                         =  2.52767e+01
>      cosab                     =  6.12323e-17
>      cosac                     =  6.12323e-17
>      cosbc                     = -1.85547e-01
>      degauss                   =  2.00000e-02
>      ecutrho                   =  4.75221e+02
>      ecutwfc                   =  5.03902e+01
>      ibrav                     = 12
>      nat                       = 53
>      nbnd                      = 480
>      nspin                     = 2
>      ntyp                      = 4
>      occupations               = "smearing"
>      smearing                  = "gaussian"
>      starting_magnetization(1) =  2.00000e-01
>      starting_magnetization(2) =  2.00000e-01
>      starting_magnetization(3) =  0.00000e+00
>      starting_magnetization(4) =  0.00000e+00
> /
> 
> &ELECTRONS
>      conv_thr         =  1.00000e-05
>      diagonalization  = "david"
>      electron_maxstep = 200
>      mixing_beta      =  1.00000e-01
>      mixing_mode      = "local-TF"
>      startingpot      = "atomic"
>      startingwfc      = "atomic+random"
> /
> 
> &IONS
>      ion_dynamics = "bfgs"
> /
> 
> &CELL
> /
> 
> K_POINTS {automatic}
>   2  2  1  0 0 0
> 
> ATOMIC_SPECIES
> N      14.00674  N.pbe-n-kjpaw_psl.1.0.0.UPF
> W     183.84000  W.pbe-spn-kjpaw_psl.1.0.0.UPF
> C      12.01070  C.pbe-n-kjpaw_psl.1.0.0.UPF
> H       1.00794  H.pbe-kjpaw_psl.1.0.0.UPF
> 
> ATOMIC_POSITIONS {angstrom}
> N       3.210864   2.727236  14.728253
> N       7.377521   2.749437  14.765113
> N       3.206885  -1.748226  14.750672
> N       7.368425  -1.739825  14.763088
> W       3.182078   4.921673  14.584931
> W       7.371236   4.911599  14.572299
> W       3.210468   0.427880  14.572328
> W       7.375435   0.429218  14.573322
> N       1.114319   5.237998  14.590842
> N       5.297509   5.253239  14.643339
> N       1.127456   0.739579  14.633097
> N       5.291670   0.741857  14.637218
> W       1.130917   3.035176  14.411937
> W       5.301873   3.012738  14.402683
> W       1.122976  -1.443293  14.413944
> W       5.288433  -1.443790  14.404037
> N       3.219452   5.199606  12.448287
> N       7.366466   5.188650  12.437657
> N       3.207970   0.704887  12.465361
> N       7.371281   0.697722  12.461527
> W       3.205392   3.068813  12.535650
> W       7.371059   3.075963  12.535225
> W       3.208342  -1.417371  12.526978
> W       7.371423  -1.415702  12.526259
> N       1.126718   3.566876  12.355498
> N       5.287387   3.570864  12.349260
> N       1.127573  -0.926625  12.347835
> N       5.290289  -0.925589  12.354591
> W       1.124934   5.676097  12.168934
> W       5.300233   5.687505  12.124192
> W       1.125212   1.183514  12.166731
> W       5.291832   1.185520  12.166744
> N       3.207882   3.506729  10.422372  0 0 0
> N       7.371462   3.506729  10.422372  0 0 0
> N       3.207882  -0.986715  10.422372  0 0 0
> N       7.371462  -0.986715  10.422372  0 0 0
> W       3.207882   5.716584  10.333236  0 0 0
> W       7.371462   5.716584  10.333236  0 0 0
> W       3.207882   1.223140  10.333236  0 0 0
> W       7.371462   1.223140  10.333236  0 0 0
> N       1.126093   6.199382  10.052766  0 0 0
> N       5.289672   6.199382  10.052766  0 0 0
> N       1.126093   1.705938  10.052766  0 0 0
> N       5.289672   1.705938  10.052766  0 0 0
> W       1.126093   3.871912  10.000000  0 0 0
> W       5.289672   3.871912  10.000000  0 0 0
> W       1.126093  -0.621532  10.000000  0 0 0
> W       5.289672  -0.621532  10.000000  0 0 0
> C       4.904436   5.088998  16.341705
> H       4.817236   4.340738  17.152672
> H       4.810563   5.831388  17.155825
> H       3.209512   2.314472  17.745318
> H       3.207503   3.080782  17.746006
> 
> 
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-- 
----------------------------------------------------------------------
Prof Nicola Marzari, Chair of Theory and Simulation of Materials, EPFL
Director, National Centre for Competence in Research NCCR MARVEL, EPFL
http://theossrv1.epfl.ch/Main/Contact http://nccr-marvel.ch/en/project


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