[Q-e-developers] Integration of EPW into QE 5

[Paolo Giannozzi]

Dear Samuel

sure I remember you, I noticed your efforts on the phonon code and I was
suspecting that you were working on EPW.
Happy to see that you made some serious progress. I also tried some time
ago to re-align EPW to the svn version, but I quickly realized that it
required more time and effort than I could afford.

While making the port, I realized that the Eliashberg lambda were very
> different between the two code (more than 25% difference). I tracked the
> problem down to the electron-phonon matrix element computed with QE [...]
>
the problematic revision seems to be the 5272 that seems to have been
> committed by you:
> r5272 | giannozz | 2008-11-05 20:25:20 +0000 (Wed, 05 Nov 2008) | 4 lines
> Only one random number generator is used everywhere ("randy", which seems
> to be the most uniform). Beware all kinds of unexpected side effects
>

> Therefore the problem seems to be related with the use of 'randy'. This
> should in theory not affect any physical quantity like the electron-phonon
> matrix element squared but it does.
>
> Do you think it is a bug or that a bug was solved by using randy ? What
> should be the physically correct quantity (before or after revision 5272)?
>

short answer: i don't know (I cannot see any good reason why
electron-phonon coefficients should be affected while phonons aren't), but
it is easier to add a bug without noticing than to remove a bug without
noticing.

As far as I know both randy and rndm should do the job, since "the job"
shouldn't be very critical in terms of randomness: fill a random matrix or
something like this, to be diagonalized to produce the irreps or something
like that. My suggestion is to try the following (no, I am not going to
try, because I haven't access to any serious machine right now and I have a
number of things to do that I cannot postpone, although most of them are
way less amusing than finding a bug):
- replace only set_irr.f90 and random_matrix.f90 with the previous versions
(you will need to put somewhere the previous random-number routines as
well), just to verify if that is the origin of the discrepancy (I cannot
see other reasons)
If so, please try the following:
- remove the "arg = randy(0)" line
- change "0" in "arg = randy(0)" to some weird number, 987654321 or
something like this
- replace "arg = randy(0)" with "call set_random_seed ( )" which should set
a really random number.
I don't know if it is really needed to have the "arg = randy(0)" line (or
equivalent initialization) in the "set_irr.f90" file (or in any other file)
instead of having it inside "random_matrix.f90". Please check if these
routines are called more than once, and if so, if they are supposed to
produce the same irreps: I vaguely remember that in order to perform some
calculations one has to save irreps to file because there is no guarantee
to have exactly the same numbers in subsequent runs (a common problem of
nondeterministic algorithms ... ).

I am sending a copy of this message to q-e-developers at qe-forge.org as well
because that is the place where this kind of questions should be discussed.
Best regards

Paolo

-- 
Paolo Giannozzi, Dept. Chemistry&Physics&Environment,
Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
Phone +39-0432-558216, fax +39-0432-558222
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