EPW Forum

When adding nosym=.true. to scf.in scf_epw.in and nscf_epw.in, example t04 no longer works. My guess is that epw finds the symmetries (as reported in epw.out) and tries to access files/data which is not there:

########################################################################################################################
# FROM IOTK LIBRARY, VERSION 1.2.0
# UNRECOVERABLE ERROR (ierr=-1)
# ERROR IN: iotk_scan_begin (iotk_scan.f90:182)
# CVS Revision: 1.23
# Tag not found
namel=DYNAMICAL_MAT_.3
########################################################################################################################

All pw.x and ph.x calculations report using no symmetry.

Without nosym=.true. all examples are working.

Is this intended behaviour and are you not allowed to set nosym to true or am I missing an input option for epw?
Note that I would like to use the nosym option.

Hello,

Why do you want to use the nosym option?

You should provide explicitly all the k-points in nscf_epw.in anyway.

Therefore not using symmetry will just make the scf calculation longer for no reason.

In principle, it should work though. Note however that this is not tested (see the espresso/test-suite/epw_* folders) so there might indeed be an issue.

Thanks for reporting it.

Best,

Samuel

Thank you for your quick reply

Two reasons:
1) Reuse old dvscf data (these calculations can take very long for larger systems) which was generated before I could make use of symmetry
2) Not all configurations have the same symmetry and it can then be difficult to compare the different modes

Perhaps it is not the k-point symmetry but the q-point symmetry which is the problem?

Hello,

The code definitely expects the use of symmetry for the q-points (dvscf) calculations.

Do you mean that you have the q-point on a grid without symmetries? So for a 6x6x6 q-grid, you did 216 q-point calculations (so you have 216 dvscf files)?

The first part of EPW will try to reconstruct the q-grid from the IBZ to the full zone. If you explicitly computed those, you can hack the code and bypass the first part.

PS: If the dvscf are very old, you should not expect backwards compatibility. If you want to use them, you will need to use an old version of the code.


Best,
Samuel

The only change I made was adding nosym=.true. to the pw.x input files (scf.in scf_epw.in nscf_epw.in) in example t04

There are indeed not 216 qpoint calculations.