[Pw_forum] xspectra calculation (wumindt2)

[Matteo Calandra]

dear Min Wu,

  the attachment do not go through the pw_forum, so I cannot
do much about it. What I can tell you is that we calculate
Co and LiCoO2 and we get a good agreement with experiments.
Maybe you can try to compare with the following paper:

Juhin et al. Phys. Rev. B 81, 115115 (2010)
http://link.aps.org/doi/10.1103/PhysRevB.81.115115

Anyway, from four message what seems to be wrong (assuming the scf
run to be correct) are the unit of the polarization vector
(assuming you're dealing with the dipolar part only and assuming
the peak you're referring to is dipolar). Did you set xcoordcrys to
the proper value (see below)?

For zhqt concerns CoO2, note that the pre-edge
of CoO2 at the Co K-edge has a quadrupolar part, so if you stick
to the dipolar calculation you want be able to reproduce the pre-edge 
peak. Note also that in the case of LiCoO2, which is probably
not so different from yours, DFT fails in the
pre-edge and you have to use DFT+U.

In particular, remember that if you don't set the variable
xcoordcrys=.false. your xepsilon and xkvec are in reduced coordinates.
 From Doc/INPUT_XSPECTRA

xkvec(1:3)        real(DP) 
DEFAULT=(1.0,0
.0,0.0)
                   coordinates of the x-ray momentum k


xepsilon(1:3)     real(DP) 
DEFAULT=(1.0,0
.0,0.0)
                   coordinates of the incident x-ray
                   polarization vector

xcoordcrys        logical 
DEFAULT=.true.
                   .true. to use crystal coordinates for
                   k and epsilon


Maybe you could try to reproduce our calculation on LiCoO2 so that
you are sure of not having any errors with the input file.
The paper is very detailed so you can compare every step in the calculation.

All the best, M.

 >Message: 6
 >Date: Mon, 18 Oct 2010 05:03:50 +0800
 >From: "wumindt2" <wumindt2 at zju.edu.cn>
 >Subject: Re: [Pw_forum] Xspectra calculation (wumindt2)
 >To: pw_forum at pwscf.org
 >Message-ID: <BJGFDAKBYOBGKSOWGQJJQOXRQCLO.wumindt2 at zju.edu.cn>
 >Content-Type: text/plain; charset="gb2312"

 >Dear Matteo,

 >I met some problem when calculating the XASs with the Xspectra.
 >I tested the example of quartz, and got the consistent result with the
 >published work.
 >Then i moved on to test another example of solid CO2, which has an 
 >isotropic
 >symmetry according to C. Brouder's paper. But the X-ray absorption 
 >spectrum
 >i got was totally wrong. The first sharp peak which is well-known 
 >existing
 >in CO2 was not found.
 >We also tested a single CO2 molecule which is still a clear-cut >example.
 >The C atom and the two O atoms are all in the Z axis. So the 
 >polarization vectors
 >of (0,0,1) and (1,0,0) are enough for this case. However, we still 
 >cannot get the
 >right XAS. To make it easier for you to understand my problem, I 
 >attached the input files of this calculation.
 >The GIPAW pseudopotentials i used were downloaded from the website of >QE.

 >Thanks a lot for the help.

 >Best regards,

 >Min Wu