[Pw_forum] Innards of ph.f90 and Matdyn.f90

[Paul M. Grant]

It is with great trepidation that I post these kinds of questions on
PWforum.  I always fear being scolded by Paolo and/or Axel for not looking
at past fora postings or within the intricacies of "the code."  (No, I
missed seeing that covered in the posting of 28 June 1914.I was otherwise
engaged).

To be serious, my present posting is motivated by the late May discussions
on "Phonon DOS in noncubic materials."

I have started some e-p ph.x calculations on several "interesting"
transition metal oxides using SCF eigenstates computed with LDA+U
implemented, of cubic and/or tetragonal rocksalt symmetry.  The end story
intention is to calculate the Eliashberg lambda.  Why would one ever want to
do this?  Think superconductivity.

Issues:
1)	 I assume ph.x can handle spin polarized states, because that's the
basis of Example02 and it seems to work OK. 
2)	 It (ph.x) also "seems to work" with  la2F and lda_plus_u set
.true., that is, it generates gammas and lambdas without barfing, for cubic
and tetragonal AFII primitive cells.  So far, so good.
3)	Question.why is it necessary to do two scf runs a la Example07, one
to set a DOS flag for separate use later, and another to set the k-point
grid for a subsequent el-ph run with ph.x?  I know there might some resource
issues for "serious" calculations, but for Example07, one scf with la2F "on"
and an MP grid of order 12, "seems to" work just fine.  At least for a
"non-serious" calculation.
4)	Next we bring C(q) from the output of ph.x to "real space" with
q2r.x.
5)	Now it gets dicey.  The following I haven't tried yet.
6)	The matdyn.x calculation to compute the frequency dependent
component of the Eliashberg kernel uses a "real space" sampling grid.in the
case of Example07 of size 19.  How is this grid determined?  It looks to me
like it's spanning some high symmetry directions within  the Al unit cell.
Is there an algorithm to calculate such for general symmetry other than Al
(or fcc) (e.g., an analog to MP sampling, or FS BZ sampling), or is such
just a user-selected symmeterized  "wet finger in the wind?"
7)	Lastly, here's where I really get confused and appreciate Dr.
Prasad's posting.  Which "dos" calculation is being referred to.the scf run
with la2F set true, or a restriction on the input to the "final" matdyn.x
run with dos set true?

What's needed is a user's manual for matdyn (sorry, Paolo), or, better yet,
three or four more examples like 07 on non-cubic but otherwise simple
symmetries and simple basis orbital sets with more than one atom per unit
cell.like MgB2!

Ciao, -Paul

Paul M. Grant, PhD
Principal, W2AGZ Technologies
Visiting Scholar, Applied Physics, Stanford University
EPRI Science Fellow (Retired)
IBM Research Staff Member Emeritus
w2agz at pacbell.net
http://www.w2agz.com
 
 


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