[Pw_forum] PHonon versions 5.0.2/5.0.3 can give very different frequencies as compared to older versions and the latest SVN builds

[Brad Malone]

Hi everyone,

I recently came across some phonon softening in a system calculated with
the 5.0.2 version of QE. This came as a surprise because, in addition to
having some physical reasons for not believing it, it was in contrast to
previous calculations I had done some time back on the same system (and
input files) with QE 4.3.2. The deviations with respect to QE version over
a 3x3x3 grid of q vectors in this system amounted to a maximal error of
13.5 cm^-1 for a particular q vector.

This previously mentioned system was fairly large, so I tried to find
simpler systems to run so that I could narrow down where the problem was
coming from. I found that many simple systems (like cubic diamond)
exhibited essentially zero error regardless of version. However, in
guessing that the problem could possibly be related to systems with
hexagonal symmetry, I chose to test the hexagonal diamond phase of Ge (4
atoms/cell) and found that over a 3x3x3 grid of q vectors, I got deviations
on the order of 95 cm^-1 for some q vectors. I actually do not think the
problem is limited to hexagonal symmetry systems however, as I've seen some
deviations in other orthorhombic systems (although they have been more
minor).

Using the hexagonal diamond system, I tested a wide variety of QE releases
and SVN versions. The problem occurs for the more recent releases of 5.0.2
and 5.0.3 (i.e., the patch doesn't fix the problem). The latest SVN version
is consistent with 5.0.1 and 5.0.0 as well as older versions tested (4.3.2
and 4.2.1). I'm not sure the exact SVN version where the problem first
occurred, but rev 9724 is the first commit which gives the "correct" phonon
frequencies.

Rev 9724 differs from 9723 as follows:

login1$ svn -r 9723:9724 diff
> Index: PHonon/PH/sgam_ph.f90
> ===================================================================
> --- PHonon/PH/sgam_ph.f90 (revision 9723)
> +++ PHonon/PH/sgam_ph.f90 (revision 9724)
> @@ -161,7 +161,8 @@
>             sym(irot) = sym(irot) .and. (abs(raq(ipol)-aq(ipol)) < 1.0d-5)
>          enddo
>       endif
> -     if (.not.minus_q) then
> +!     if (.not.minus_q) then
> +     if (sym(irot).and..not.minus_q) then
>          raq = - raq
>          minus_q = eqvect (raq, aq, zero, accep)
>       endif


Anyway, hope these tests are useful. Unless I've made a mistake somewhere,
I think it suggests that it's extremely dangerous to do phonon calculations
with 5.0.2 or 5.0.3 and you should either stick to 5.0.1 or svn revision
9724 or later.

Best,
Brad Malone
Harvard University
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