[Pw_forum] Wrong representation in 5.0

[Lorenzo Paulatto]

On Mon, Jul 23, 2012 at 1:57 PM, Davide Ceresoli <
davide.ceresoli at istm.cnr.it> wrote:

> I can supply input coordinates and output files to whoever knows
> how to fix this problem with PH in version 5.0.
>
>
Hi Davide,
the good news is that with espresso 4.x the calculation would run; the bad
news is that it would produce random garbage.

The error arises because of a newly introduced check which verifies that
the irreducible representations are actually representations. The origin of
the problem is the method used to find the irreducible representations: a
random pseudo-dynamical matrix is generated, than it is symmetrized and
diagonalized. The resulting eigenvectors are used as phonon displacement
patters; if the eigenvalues (pseudo-frequencies) of two patterns are
degenerate, also the real frequencies will be.

The problem is that sometimes a bit of numerical error in the
diagonalization makes two modes appear as not degenerate when they should
be, which in turn causes the irreducible representation to not be a
representation. We could increase the threshold a bit, but then there would
be a higher risk of getting accidental degeneration, causing the irr.reps.
to not be irreducible...

As a temporary workaround you can fiddle a bit with the threshold at line
258 of PH/set_irr.f90 or with how the random matrix in generated in
PH/random_matrix.f90 (i.e. increasing the diagonal component). I would also
ask you to send me the coordinates, to use as a test case, as I'm trying to
find a more reliable way to generate the matrix.

bests

-- 
Lorenzo Paulatto IdR @ IMPMC/CNRS & Université Paris 6
phone: +33 (0)1 44275 084 / skype: paulatz
www:   http://www-int.impmc.upmc.fr/~paulatto/
mail:  23-24/4é16 Boîte courrier 115, 4 place Jussieu 75252 Paris Cédex 05
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