[Pw_forum] conversence problem in vc-relax (Atom Mass in relaxation)

Sandro Scandolo scandolo at ictp.it
Mon Sep 10 22:23:14 CEST 2007


Dear Yukihiro,

while a higher accuracy may be required in some exceptional cases, 
converging the stress tensor (or the average pressure) within 1 kbar of 
the target value is already a physically sound achievement. The error 
DeltaV in volume that one is making with a DeltaP~+-1 kbar error in the 
pressure is given by DeltaV/V=DeltaP/Bulk_modulus. With the exception of 
molecular or very soft solids, the bulk modulus of the vast majority of 
solids is typically > 10 GPa, or 100 kbar, so your volume is likely to 
be converged to better than 1% already....

This of course should not prevent you from asking a higher accuracy, if 
required, in which case Cesar's comments are likely to be helpful.

Regards,
Sandro

Yukihiro Okuno wrote:
> Dear PWScf Users.
>
> I'm Yukihiro Okuno.
>
> I had posted about the Atom Mass in vc-relax, and thank you for the responces.
> We set same atom mass 20.0 ( average of atom mass), and do vc-relax calculation as
> the following input file,
>
>
> &control
>     calculation  = 'vc-relax',
>     restart_mode = 'from_scratch',
>     pseudo_dir   = '/pseudo/',
>     outdir       = './', 
>     prefix='vc-test'
>     nstep = 100
>     etot_conv_thr = 1.0D-5
>     forc_conv_thr = 1.0D-4
>     dt = 80.0
>     tstress = .true.
>     tprnfor = .true.
>   /
>  &system
>     ibrav=6
>     celldm(1)=6.83994507291
>     celldm(3)=1.09801321247
>     nat=5
>     ntyp=3
>     nbnd=26
>     ecutwfc=60.0
>     ecutrho = 600.0
>     occupations = 'fixed'
>     degauss=0.00
>  /
>  &electrons
>     conv_thr = 1e-10,
>     mixing_beta=0.3,
>  /
> &IONS
>  ion_dynamics='damp' 
>  bfgs_ndim=3
>  pot_extrapolation = "second_order"
>  wfc_extrapolation = "second_order"
> /
> &CELL
>   cell_dynamics  = 'damp-w',
>   press = 0.0
>   wmass = 0.02
> /
>
>
> But the calculation is not conversed and oscillate. The system has only five atoms 
> and simple P4mm perovskite structure. 
> I had change wmass as default value, but the convergence is not achieved.
> Is it too hard to converse to set  forc_conv_thr = 1.0D-4 in VC-relax calculation ? 
> ( I want to get severe internal atomic coordinate, and forc_conv_thr=1.0D-3 is not 
> sufficient.) Simple atomic internal relaxation, I can get converged result. 
>
>
> The stress and total force of output file is as follow.
>
> stress values ,,
>
>     entering subroutine stress ...
>           total   stress  (Ry/bohr**3)                   (kbar)     P=   -0.13
>      entering subroutine stress ...
>           total   stress  (Ry/bohr**3)                   (kbar)     P=   -2.09
>      entering subroutine stress ...
>           total   stress  (Ry/bohr**3)                   (kbar)     P=   -1.17
>      entering subroutine stress ...
>           total   stress  (Ry/bohr**3)                   (kbar)     P=   -0.60
>      entering subroutine stress ...
>           total   stress  (Ry/bohr**3)                   (kbar)     P=    0.07
>      entering subroutine stress ...
>           total   stress  (Ry/bohr**3)                   (kbar)     P=    0.37
>      entering subroutine stress ...
>           total   stress  (Ry/bohr**3)                   (kbar)     P=    0.84
>      entering subroutine stress ...
>           total   stress  (Ry/bohr**3)                   (kbar)     P=    0.95
>      entering subroutine stress ...
>           total   stress  (Ry/bohr**3)                   (kbar)     P=    1.07
>      entering subroutine stress ...
>           total   stress  (Ry/bohr**3)                   (kbar)     P=    1.22
>      entering subroutine stress ...
>           total   stress  (Ry/bohr**3)                   (kbar)     P=    1.36
>      entering subroutine stress ...
>           total   stress  (Ry/bohr**3)                   (kbar)     P=    1.47
>      entering subroutine stress ...
>           total   stress  (Ry/bohr**3)                   (kbar)     P=    1.55
>      entering subroutine stress ...
>           total   stress  (Ry/bohr**3)                   (kbar)     P=    0.96
>      entering subroutine stress ...
>           total   stress  (Ry/bohr**3)                   (kbar)     P=    0.65
>      entering subroutine stress ...
>           total   stress  (Ry/bohr**3)                   (kbar)     P=    0.39
>      entering subroutine stress ...
>           total   stress  (Ry/bohr**3)                   (kbar)     P=    0.20
>      entering subroutine stress ...
>           total   stress  (Ry/bohr**3)                   (kbar)     P=    0.04
>      entering subroutine stress ...
>           total   stress  (Ry/bohr**3)                   (kbar)     P=   -0.08
>      entering subroutine stress ...
>           total   stress  (Ry/bohr**3)                   (kbar)     P=   -0.13
>      entering subroutine stress ...
>           total   stress  (Ry/bohr**3)                   (kbar)     P=   -0.13
>      entering subroutine stress ...
>           total   stress  (Ry/bohr**3)                   (kbar)     P=   -0.63
>      entering subroutine stress ...
>           total   stress  (Ry/bohr**3)                   (kbar)     P=   -0.89
>      entering subroutine stress ...
>           total   stress  (Ry/bohr**3)                   (kbar)     P=   -1.05
>      entering subroutine stress ...
>           total   stress  (Ry/bohr**3)                   (kbar)     P=   -1.04
>      entering subroutine stress ...
>           total   stress  (Ry/bohr**3)                   (kbar)     P=   -1.13
>
>
> and  The total force, 
>
>      Total force =     0.000512     Total SCF correction =     0.000057
>      Total force =     0.000499     Total SCF correction =     0.000063
>      Total force =     0.000472     Total SCF correction =     0.000024
>      Total force =     0.000411     Total SCF correction =     0.000022
>      Total force =     0.000385     Total SCF correction =     0.000053
>      Total force =     0.000527     Total SCF correction =     0.000012
>      Total force =     0.000487     Total SCF correction =     0.000188
>      Total force =     0.000572     Total SCF correction =     0.000010
>      Total force =     0.000537     Total SCF correction =     0.000016
>      Total force =     0.000484     Total SCF correction =     0.000016
>      Total force =     0.000440     Total SCF correction =     0.000007
>      Total force =     0.000407     Total SCF correction =     0.000017
>      Total force =     0.000369     Total SCF correction =     0.000016
>      Total force =     0.000391     Total SCF correction =     0.000045
>      Total force =     0.000552     Total SCF correction =     0.000015
>      Total force =     0.000569     Total SCF correction =     0.000130
>      Total force =     0.000683     Total SCF correction =     0.000026
>      Total force =     0.000623     Total SCF correction =     0.000025
>      Total force =     0.000544     Total SCF correction =     0.000020
>      Total force =     0.000404     Total SCF correction =     0.000015
>      Total force =     0.000280     Total SCF correction =     0.000028
>      Total force =     0.000219     Total SCF correction =     0.000011
>      Total force =     0.000348     Total SCF correction =     0.000039
>      Total force =     0.000552     Total SCF correction =     0.000015
>      Total force =     0.000688     Total SCF correction =     0.000019
>      Total force =     0.000741     Total SCF correction =     0.000013
>      Total force =     0.000723     Total SCF correction =     0.000013
>      Total force =     0.000638     Total SCF correction =     0.000018
>      Total force =     0.000504     Total SCF correction =     0.000017
>      Total force =     0.000368     Total SCF correction =     0.000029
>      Total force =     0.000256     Total SCF correction =     0.000020
>      Total force =     0.001173     Total SCF correction =     0.000016
>      Total force =     0.001841     Total SCF correction =     0.000271
>      Total force =     0.001924     Total SCF correction =     0.000220
>      Total force =     0.002328     Total SCF correction =     0.000057
>      Total force =     0.002448     Total SCF correction =     0.000019
>      Total force =     0.002318     Total SCF correction =     0.000016
>      Total force =     0.002004     Total SCF correction =     0.000018
>      Total force =     0.001547     Total SCF correction =     0.000022
>      Total force =     0.001029     Total SCF correction =     0.000026
>      Total force =     0.000669     Total SCF correction =     0.000015
>      Total force =     0.002834     Total SCF correction =     0.000206
>      Total force =     0.003057     Total SCF correction =     0.000170
>      Total force =     0.001864     Total SCF correction =     0.000273
>      Total force =     0.001107     Total SCF correction =     0.000101
>      Total force =     0.000870     Total SCF correction =     0.000021
>      Total force =     0.000355     Total SCF correction =     0.000068
>      Total force =     0.000613     Total SCF correction =     0.000114
>      Total force =     0.000484     Total SCF correction =     0.000038
>      Total force =     0.000338     Total SCF correction =     0.000180
>      Total force =     0.000372     Total SCF correction =     0.000125
>      Total force =     0.000460     Total SCF correction =     0.000028
>      Total force =     0.000482     Total SCF correction =     0.000014
>      Total force =     0.000528     Total SCF correction =     0.000018
>      Total force =     0.000652     Total SCF correction =     0.000008
>      Total force =     0.000729     Total SCF correction =     0.000017
>      Total force =     0.000740     Total SCF correction =     0.000013
>      Total force =     0.000684     Total SCF correction =     0.000005
>      Total force =     0.000573     Total SCF correction =     0.000005
>      Total force =     0.000446     Total SCF correction =     0.000012
>      Total force =     0.000369     Total SCF correction =     0.000010
>      Total force =     0.000408     Total SCF correction =     0.000005
>      Total force =     0.000602     Total SCF correction =     0.000044
>      Total force =     0.000897     Total SCF correction =     0.000015
>      Total force =     0.001145     Total SCF correction =     0.000020
>      Total force =     0.001247     Total SCF correction =     0.000004
>      Total force =     0.001348     Total SCF correction =     0.000050
>
> Sincerely.
>
>
> ----
> Yukihiro Okuno  okunoyukihiro2000 at yahoo.co.jp
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