[Pw_forum] Additional informations about XSpectra - Peaks, at negative energies?

Matteo Calandra matteo.calandra at impmc.jussieu.fr
Fri Aug 20 09:43:33 CEST 2010

Dear Christian,

   indeed it is related to the cut_occ_states option. If this option is 
false you also see the occupied states at negative energies.
This is very useful in many case. If now you want to cut the occupied
states, simply rerun xspectra using the cut_occ_states=.true. option and 
the spectrum will be smoothly cut out at the e_f you have specified.
Note also that you don't need to perform again the full lanczos process
but only to replot the curve, namely use the option
in the namelist
provided you have kept the .sav file with all the informations on the
previously performed lanczos process, namely the a and b coefficients.
If you give a look at the diamond example in the 
examples/XSpectra_examples of the distribution then you will
find all the informations you need.

Finally, if the cut_occ_states option takes too much time then there are
other parameters to tune that can drastically reduce the time needed.
To understand how cut_occ_states works you can look at
  Phys. Rev. B 54, 7334–7349 (1996)
sections III A and III b.
However I suggest you do not touch the parameters related to 
cut_occ_states unless you know what you do.

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

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