[Pw_forum] Additional informations about XSpectra

Matteo Calandra matteo.calandra at impmc.jussieu.fr
Wed Aug 18 11:20:08 CEST 2010

Dear Christian,

  here the answers to your questions:

 > (1) is the energy range (xemin,xemax) referred to the Fermi energy
 > of the preliminar scf calculation?

No, it is referred to the Fermi level given in input  in the xspectra 
file (input variable ef_r in the input_xspectra namelist. This means 
that if you do not specify the Fermi level then ef_r=0.0 and
they are referred to the energy zero of the preliminary scf run.

 > (1) is there a special convention for the components of the wavevector
 > and of the polarization vector? The defaults are (1,0,0) and (0,1,0);
 > shouldn't they be orthogonal?

This depends on the variable xcoordcrys. If the variable xcoordcrys=.true.
then xkvec and xepsilon are the crystalline components of the wavevector
and the polarization vector with respect to the direct lattice vectors 
as chosen by QE. On the contrary, if xcoordcrys=.false. then they are 
the components with respect to the cartesian axes (in direct space).
Note that also the xkvec components are in direct space as this is
customary in the XAS community.
xkvec and xepsilon should of course be orthogonal. If they are not, then 
the code gives you a warning in output but it does not stop.
The reason is that if you do a dipolar calculation only, then xkvec is

 > (3) last line of input, after the $cut_occ section
 > (question already posted on past 2/7 in this forum)

it is the number of k-point necessary to calculate the
XANES cross section. In practice you can chose to calculate
the charge density with a given number of k-points and then
the XANES with a larger one. Generally the convergence
of the xanes cross section with respect to k-points is slower
thant that of the charge density.
This is similar to what you do
with a density of states calculation, namely you first calculate
the charge density with Nk k points and then the dos
with a much larger Nk' grid.

All the best, M.


* * * *
Matteo Calandra, Charge de Recherche (CR1)
Institut de Minéralogie et de Physique des Milieux Condensés de Paris
Université Pierre et Marie Curie, tour 16, case 115
4 Place Jussieu, 75252 Paris Cedex 05 France
Tel: +33-1-44 27 52 16       Fax: +33-1-44 27 37 85

More information about the users mailing list