<div class="gmail_quote">On Mon, Jul 23, 2012 at 1:57 PM, Davide Ceresoli <span dir="ltr"><<a href="mailto:davide.ceresoli@istm.cnr.it" target="_blank">davide.ceresoli@istm.cnr.it</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
I can supply input coordinates and output files to whoever knows<br>
how to fix this problem with PH in version 5.0.<br><br></blockquote><div><br></div><div>Hi Davide,</div><div>the good news is that with espresso 4.x the calculation would run; the bad news is that it would produce random garbage.</div>
<div><br></div><div>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.</div>
<div><br></div><div>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...</div>
<div><br></div><div>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.</div>
<div><br></div><div>bests</div></div><div><br></div>-- <br><div>Lorenzo Paulatto IdR @ IMPMC/CNRS & Université Paris 6</div><div>phone: +33 (0)1 44275 084 / skype: paulatz</div><div>www: <a href="http://www-int.impmc.upmc.fr/~paulatto/" target="_blank">http://www-int.impmc.upmc.fr/~paulatto/</a></div>
<div>mail: 23-24/4é16 Boîte courrier 115, 4 place Jussieu 75252 Paris Cédex 05</div><br>