# [Pw_forum] Problem: different output with identical input

Ilya Ryabinkin igryabinkin at gmail.com
Wed Dec 28 13:53:43 CET 2016

```Iterative solution of KS equations (with a given threshold) limits the
number of significant figures in both energy and properties values.
The total energy is less sensitive than property values are. What
happens is the following: you *gradients* have limited accuracy --
beyond threshold the digits are largely random. These errors propagate
and accumulate throughout geometry optimization.

--
Ilya

On Wed, Dec 28, 2016 at 7:45 AM, Pablo García Risueño
<garcia.risueno at gmail.com> wrote:
> 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

--
*******************************************************
Ilya Ryabinkin
Postdoctoral Scholar
Physical and Environmental Sciences
University of Toronto Scarborough
http://www.utsc.utoronto.ca/~aizmaylov/Members.html
*******************************************************

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