[gpaw-users] Error from transformers.py when restarting a calculation

[Ask Hjorth Larsen]

Hi Peder

On Wed, 19 May 2010, Peder Olesen wrote:

> Hi all,
> 
> I'm attempting to calculate some electron-phonon coupling matrix elements,
> and for that purpose I need to reuse a calculator. However, when I produce
> the file calcUC.gpw using the script:
>
>       from ase import *
>       from gpaw import *
>       a = 3.5691245496
>       b = a/2.
>
>       bulkUC = Atoms('C2',positions=([0,0,0],[b/2,b/2,b/2]),
>       cell=([0,b,b],[b,0,b],[b,b,0]),pbc=False)
>       bulkUC.set_pbc(True)
>
>       calcUC = GPAW(kpts=(20,20,20), xc='PBE', h=0.2,
>       txt='calcUC.out', nbands=-1)
>       bulkUC.set_calculator(calcUC)
>       e=bulkUC.get_potential_energy()
>       calcUC.write('calcUC.gpw')
> 
> 
> ...and subsequently attempt to load it using any script with the lines
> 
> from gpaw import *
> calcUC = GPAW('calcUC.gpw', txt='calcUC.out')
> 
> ...I recieve the following error whenever the the above lines are in a
> script using n>23 cores
>
>       Traceback (most recent call last):
>         File "mat_elem.py", line 2, in ?
>           calcUC = GPAW('calcUC.gpw', txt='calcUC.out')
>         File "/home/camp/s082073/gpaw/gpaw/paw.py", line 116, in
>       __init__
>           self.initialize()
>         File "/home/camp/s082073/gpaw/gpaw/paw.py", line 571, in
>       initialize
>           magmom_a, par.hund)
>         File "/home/camp/s082073/gpaw/gpaw/density.py", line 74, in
>       initialize
>           allocate=False)
>         File "/home/camp/s082073/gpaw/gpaw/transformers.py", line 143,
>       in Transformer
>           t = _Transformer(gdin, gdout, nn, dtype, allocate)
>         File "/home/camp/s082073/gpaw/gpaw/transformers.py", line 48,
>       in __init__
>           assert ((gdin.n_c[0] + 2 * nn - 1) * (gdin.n_c[1] + 2 * nn -
>       1) <=
>       AssertionError
> 
> 
> If I do a dry-run=n with n=23 i get
>
>       Traceback (most recent call last):
>         File "mat_elem.py", line 2, in ?
>           calcUC = GPAW('calcUC.gpw', txt='calcUC.out')
>         File "/home/camp/s082073/gpaw/gpaw/paw.py", line 116, in
>       __init__
>           self.initialize()
>         File "/home/camp/s082073/gpaw/gpaw/paw.py", line 518, in
>       initialize
>           domain_comm, parsize)
>         File "/home/camp/s082073/gpaw/gpaw/grid_descriptor.py", line
>       124, in __init__
>           raise ValueError('Grid too small!')
>       ValueError: Grid too small!
> 
> 
> whereas n<23 gives no problems at all. I would like to use n>23 cores for
> some subsequent operations. Obviously the problem has something to do with
> parallelization, but does anyone know in greater detail what the particular
> reason the above errors might be, and what I should do to avoid them?
> 
> Thanks in advance,
> Peder J. Olesen

Sounds like GPAW isn't too happy about using many CPUs for a small 
calculation.  Try starting GPAW like this:

from gpaw.mpi import world

calc = GPAW(..., communicator=[world.rank], ...)

This will make the calculator exist individually and independently on each 
process (you can also use some if-statements to make sure it only gets 
created on one of them or so).  The communicator keyword can be set to any 
set of ranks as necessary.

Regards
Ask