[Pw_forum] Problem: different output with identical input

Stefano de Gironcoli degironc at sissa.it
Wed Dec 28 16:29:18 CET 2016 

Dear Pablo Garcia Risueño

    I'm not sure I understand the problem.
    the two final positions for ecut=30 are identical to my eyes.
    the two final positions for ecut=60 differ by about 10-5 A. !
    If the property you are interested in depends so strongly on the  
atomic positions you better learn how to live with it. Even assuming  
that you are using perfect xc functional, perfect pseudopotentials and  
converged cutoff and k-points (which likely you don't), your  
calculation is neglecting zero point energy, thermal expansion,  
quantum nature of H motion, ...

    As for your specific question. In principle if you tighten enough  
the convergence the minima should tend to converge toward a common  
value. However the stopping criteria will make the two calculations  
stop as soon they are satisfied. In your case as soon as each force  
component is lower than 10-6 Ry/au and the energy does not change more  
that 10-8. Did the calculation complete ? 10-8 for the energy is  
easily reachable but 10-6 for the forces looks to me very demanding.

  Anyhow... even if the relaxation did complete successfully two  
equivalent calculations may still differ. If you run your calculations  
with exactly the same input on exactly the same machine with exactly  
the same mpi setup the results should be identical ..but why doing  
twice exactly the same calculation ?. If however you change anything  
in the input (Davidson vs CG, starting wavefunction options, mixing  
mode, BFGS-related parameters) or parallel computational setup the  
relaxation history will be slightly different and the final  
configurations will differ by a certain amount allowed by the  
tolerance defined by your stopping criteria.

  HTH

stefano

Quoting Pablo García Risueño <garcia.risueno at gmail.com>:

> Dear professor
>
> Thank you very much for your reply. The differences are important in this
> case, my final calculated quantities are very sensitive to these optimized
> positions. Should I change any of the ***conv_thr variables, or other
> variable, to have the same result for same inputs?
>
> Is there any random number in the algorithm of relax calculations, can we
> be sure that different outputs with the same input are not due to any bug?
>
> Thank you very much. Best regards.
>
> 2016-12-28 14:10 GMT+01:00 Paolo Giannozzi <p.giannozzi at gmail.com>:
>
>> The differences you find are very small. Nothing to worry about in my
>> opinion.
>>
>> By the way: Davidson diagonalization is typically faster than CG; do not
>> specify incompatible options in K_POINTS (gamma or automatic, not both;
>> gamma should be used unless you have a good reason not to)
>>
>> Paolo
>>
>> Il 28/dic/2016 01:46 PM, "Pablo García Risueño" <garcia.risueno at gmail.com>
>> ha scritto:
>> >
>> > Dear Espresso community
>> >
>> > I have one problem that is important for me; it is somewhat surprising.
>> I run geometry optimization (relax) calculations with pw.x with identical
>> input files, and I obtain rather different final coordinates. The problem
>> does not happen if ecutwfc is 30, but it does appear for cutoffs of 60, 80
>> or 90. Below one can see the exact input file, and examples of the
>> difference between the final coordinates for both runs (both run with
>> identical input) for given cutoffs.
>> >
>> > Could anybody give me a clue on the origin of the problem, and how to
>> solve it?
>> >
>> > Thank you very much. Best regards.
>> >
>> >
>> >
>> > Input file:
>> >
>> >
>> >
>> > &CONTROL
>> >
>> >     calculation = 'relax',
>> >
>> >     restart_mode = 'from_scratch',
>> >
>> >     prefix='',
>> >
>> >     outdir = './',
>> >
>> >     pseudo_dir = '/path_xxx/PP/',
>> >
>> >     forc_conv_thr = 1.0D-6 ,
>> >
>> >     etot_conv_thr = 1.0D-8 ,
>> >
>> >  /
>> >
>> > &system
>> >
>> >     ibrav = 0, a=18.0,
>> >
>> >     nat= 26, ntyp= 2,
>> >
>> >     ecutwfc = 30d0,
>> >
>> >     nbnd = 100,
>> >
>> > /
>> >
>> > &electrons
>> >
>> >     conv_thr = 1.0e-9,
>> >
>> >     mixing_beta = 0.7,
>> >
>> >     mixing_mode = 'plain',
>> >
>> >     diagonalization = 'cg'
>> >
>> > /
>> >
>> > &IONS
>> >
>> > /
>> >
>> >
>> >
>> > ATOMIC_SPECIES
>> >
>> > C   12.0107   C.pz-vbc.UPF
>> >
>> > H   1.007825035  H.pz-vbc.UPF
>> >
>> >
>> >
>> > ATOMIC_POSITIONS { angstrom }
>> >
>> > C  8.891700e+00     8.891700e+00     8.891700e+00
>> >
>> > C  9.783400e+00     9.783400e+00     8.000000e+00
>> >
>> > C  9.783400e+00     8.000000e+00     9.783400e+00
>> >
>> > C  8.000000e+00     9.783400e+00     9.783400e+00
>> >
>> > C  8.891700e+00     1.067510e+01     1.067510e+01
>> >
>> > C  1.067510e+01     1.067510e+01     8.891700e+00
>> >
>> > C  1.067510e+01     8.891700e+00     1.067510e+01
>> >
>> > C  9.783400e+00     1.156680e+01     9.783400e+00
>> >
>> > C  9.783400e+00     9.783400e+00     1.156680e+01
>> >
>> > C  1.156680e+01     9.783400e+00     9.783400e+00
>> >
>> > H  8.391700e+00     8.391700e+00     8.391700e+00
>> >
>> > H  7.500000e+00     1.028340e+01     9.283400e+00
>> >
>> > H  7.500000e+00     9.283400e+00     1.028340e+01
>> >
>> > H  9.283400e+00     1.028340e+01     7.500000e+00
>> >
>> > H  1.028340e+01     9.283400e+00     7.500000e+00
>> >
>> > H  9.283400e+00     7.500000e+00     1.028340e+01
>> >
>> > H  1.028340e+01     7.500000e+00     9.283400e+00
>> >
>> > H  9.283400e+00     1.206680e+01     9.283400e+00
>> >
>> > H  1.206680e+01     9.283400e+00     9.283400e+00
>> >
>> > H  9.283400e+00     9.283400e+00     1.206680e+01
>> >
>> > H  8.391700e+00     1.117510e+01     1.117510e+01
>> >
>> > H  1.117510e+01     1.117510e+01     8.391700e+00
>> >
>> > H  1.117510e+01     8.391700e+00     1.117510e+01
>> >
>> > H  1.028340e+01     1.206680e+01     1.028340e+01
>> >
>> > H  1.028340e+01     1.028340e+01     1.206680e+01
>> >
>> > H  1.206680e+01     1.028340e+01     1.028340e+01
>> >
>> >
>> >
>> >
>> >
>> > CELL_PARAMETERS {cubic}
>> >
>> >  1.00  0.00  0.00
>> >
>> >  0.00  1.00  0.00
>> >
>> >  0.00  0.00  1.00
>> >
>> >
>> >
>> > K_POINTS {gamma} {automatic}
>> >
>> > 1 1 1  0 0 0
>> >
>> >
>> >
>> >
>> >
>> > The program is run with
>> >
>> >
>> >
>> > mpirun -np 32 /path_xxx/pw.x
>> >
>> >
>> >
>> > Final coordinates: The first two rows for ecutwfc=30 for two runs with
>> identical input are:
>> >
>> > ATOMIC_POSITIONS (angstrom)
>> > C  8.898432121  8.898432121  8.898432121
>> > C  9.783306562  9.783306562  8.019670107
>> >
>> > ATOMIC_POSITIONS (angstrom)
>> > C  8.898432121  8.898432121  8.898432121
>> > C  9.783306562  9.783306562  8.019670109
>> >
>> >
>> > Final coordinates: The first two rows for ecutwfc=60 for two runs with
>> identical input are:
>> >
>> > ATOMIC_POSITIONS (angstrom)
>> > C  8.904988579  8.904988579  8.904988579
>> > C  9.783426086  9.783426086  8.031809869
>> >
>> > ATOMIC_POSITIONS (angstrom)
>> > C  8.904962246  8.904962246  8.904962246
>> > C  9.783425401  9.783425401  8.031847251
>> >
>> > --
>> > --
>> >
>> > Dr. Pablo García Risueño
>> >
>> > Institut für Physikalische Chemie, Universität Hamburg, Grindelallee
>> 117, 20146 Hamburg
>> >
>> > Tel. +49 040 42 83 84 82 7
>> >
>> > _______________________________________________
>> > Pw_forum mailing list
>> > Pw_forum at pwscf.org
>> > http://pwscf.org/mailman/listinfo/pw_forum
>>
>>
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>
>
>
> --
> --
>
> Dr. Pablo García Risueño
>
> Institut für Physikalische Chemie, Universität Hamburg, Grindelallee 117,
> 20146 Hamburg
>
> Tel. +49 040 42 83 84 82 7

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