[Pw_forum] Additional informations about XSpectra - Peaks, at negative energies?
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 xonly_plot=.true., in the namelist _xspectra 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 Min?ralogie et de Physique des Milieux Condens?s de Paris Universit? 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 http://www.impmc.jussieu.fr/~calandra
[Pw_forum] Additional informations about XSpectra - Peaks at negative energies?
Thanks to Matteo Calandra for his answers. Regarding the Fermi level, I have now an additional question based on an example we are dealing with. With a preliminar run of XSpectra I see a Fermi energy of -0.259, so I use this value for ef_r in following calculations. If I include negative energies (say -30 eV, with respect to this one) in the input file then in the resulting spectrum I see a *sharp* peak at about -5 eV. Now, if levels at negative energy should be all occupied, how can it result such an evident absorption peak? Is it related to the "cut_occ_states" option? By default it is .false. What does this mean exactly? Thanks again for your help. Cristian > (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. -- ___ Cristian Degli Esposti Boschi CNR, CNISM, Unita' di Ricerca di Bologna, c/o Dipartimento di Fisica, Universita' di Bologna viale Berti-Pichat, 6/2, 40127, Bologna, Italia tel. ++39 051 2095114 fax ++39 051 2095113 e-mail: cristian.degliesposti -AT- unibo.it web:http://www.df.unibo.it/fismat/theory ___
