[Pw_forum] Possible bug(?) in q2r.x when using FFT routine in ESSL

[Jiseok Kim]

Dear, QE users

I have been struggling for a while to find out what's going on with q2r.x
The problem was that I could not get rid of the error messages below when
doing q2r.x.

-------------------------------------------------
DCFT3 : 2538-2030
The transform length (ARG NO. 9) is not an allowed value. The next higher
allowed value is (2).
-------------------------------------------------

I've compiled QE-v5.0.1. on AIX6.1(parallel machine) with 'mpxlf' with ESSL
routines.
So the 'q2r.x' calls 'cfft3d' and then the 'cfft3d' calls 'dcft3' in ESSL
when performing FFT.

The problem is this.
The 'dcft3' in ESSL only accepts certain values of transforming length as
shown in,

-------------------------------------------------
http://publib.boulder.ibm.com/infocenter/clresctr/vxrx/index.jsp?topic=%2Fco
m.ibm.cluster.essl.v5r1.essl100.doc%2Fam501_hscft3.htm
-------------------------------------------------

so the FFT dimensions (nx, ny, nz), which is read from outputs by ph.x,
should be the values in the table.
Otherwise, the 'dcft3' crashes.

The FFT dimensions (nx, ny, nz) in q2r.x are equivalent to the (nq1, nq2,
nq3) in ph.x so that the Monkhorst-pack grid for ph.x should be limited by
the values in the table when using the ESSL.

For example, the Monkhorst-pack grid (16x16x1) for q-points in the case of
two-dimensional supercell, such as graphene, cannot be accepted by the
'dcft3' so we have to avoid using the nq3=1.
(For some reason, the 'dcft3' does not allow odd number of FFT dimensions..)

So here is a question.
I wonder if I can use the different Monkhorst-pack grid for pw.x and ph.x
when calculating a phonon dispersion.
For example, can I use the (16x16x1) k-points for pw.x and (16x16x2)
q-points for ph.x. in the case of graphene or some two dimensional supercell
?
I guess it should not be matter but I just want to be sure ..

Thank you.

Sincerely,

JISEOK KIM, Ph.D.
Postdoctoral Research Associate
Department of Materials Science and Engineering
University of Texas at Dallas
800 W. Campbell Rd. RL10
Richardson, TX 75080
(413)386-6285


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