[Pw_forum] Grid differences between machines - why?

[Stefano de Gironcoli]

The code choose automatically the smallest grid that is compatible with 
the specified cutoff AND is a legal value for the FFT library used.

Different FFT libraries have different allowed FFT dimensions and this 
may give different numbers on different machines.

This also makes the energy slightly different on different machines: the 
only piece that depends explicitely on the grid parameters is the XC 
part of the energy that is computed numerically on the grid. The 
difference should be small, expecially for LDA calculations.

In order to make an exact comparison between different machines one can 
specify in the &system name list common values for nr1, nr2,nr3 (and if 
needed nr1s, nr2s, nr3s).
Then the results on different machines should be exactly equivalent.

Stefano de Gironcoli

Konstantin Kudin wrote:

> Here is the situation. I am running PWSCF 2.0.0 on an
>Intel box and on an IBM SP3 (?). The input files are
>identical. Intel's code is serial, IBM's is mpi,
>32-bit executable.
>
>On the IBM SP box I get:
>     G cutoff = 1309.2944  (  99231 G-vectors)     FFT
>grid: ( 80, 80, 80)
>     G cutoff = 1047.4356  (  71005 G-vectors)  smooth
>grid: ( 66, 66, 66)
>
>On the Intel:
>     G cutoff = 1309.2944  (  99231 G-vectors)     FFT
>grid: ( 75, 75, 75)
>     G cutoff = 1047.4356  (  71005 G-vectors)  smooth
>grid: ( 72, 72, 72)
>
> 
> Why do I get different grids? They also lead to some
>differences in the energy ...
>
> Kostya
>
>__________________________________
>Do you Yahoo!?
>Yahoo! Search - Find what you’re looking for faster
>http://search.yahoo.com
>_______________________________________________
>Pw_forum mailing list
>Pw_forum at pwscf.org
>http://www.democritos.it/mailman/listinfo/pw_forum
>  
>