XSPECTRA --------- 2009: First version of XSpectra by Christos Gougoussis, Matteo Calandra, Ari P. Seitsonen and Francesco Mauri 2014: Restyling of I/O, by Delphine Cabaret and Nadejda Mas 2015: L23 edge XAS calculation by O. Bunau and M. Calandra ----------------------------------------------------------------------- The theoretical approach on which XSpectra is based was described in: L23 edges, O. Bunau and M. Calandra Projector augmented wave calculation of x-ray absorption spectra at the L2,3 edges Phys. Rev. B 87, 205105 (2013) K/L1-edge, C. Gougoussis, M. Calandra, A. P. Seitsonen, F. Mauri, "First principles calculations of X-ray absorption in an ultrasoft pseudopotentials scheme: from $\alpha$-quartz to high-T$_c$ compounds", Phys. Rev. B 80, 075102 (2009) M. Taillefumier, D. Cabaret, A. M. Flank, and F. Mauri "X-ray absorption near-edge structure calculations with the pseudopotentials: Application to the K edge in diamond and αalpha-quartz" Phys. Rev. B 66, 195107 (2002) You should cite these three works in all publications using this software. The implementation of the DFT+U approximation and its application to K-edge XAS in NiO was performed in: C. Gougoussis, M. Calandra, A. Seitsonen, Ch. Brouder, A. Shukla, F. Mauri " Intrinsic charge transfer gap in NiO from Ni K -edge x-ray absorption spectroscopy", Phys. Rev. B 79, 045118 (2009) If you use DFT+U, you should cite this work too. Finally you should cite properly the Quantum ESPRESSO distribution. ----------------------------------------------------------------------- XSpectra is a post-processing tools that relies on the output (the charge density) of the PWscf code (pw.x). Thus a scf calculation needs to be done before running xspectra.x. To simulate core-hole effects, a pseudopotential with a hole in the s state (1s for K-edges, 2s for L1-edges, 2p1/2 for L2-edges, 2p3/2 for L3-edges) needs to be generated for the absorbing atom. Some of these pseudopotentials are available in the XSpectra examples directory, some others are available on the pseudopotential web-page at www.quantum-espresso.org/ with the label "*star1s*_gipaw*" for K-edges, "*star2s*_gipaw*" for L1-edges and so on. The self-consistent calculation is then performed on a supercell including the absorbing atom. The size of the supercell needs to be verified from system to system, since fairly large supercells are necessary for convergence. If core-hole effects need not to be taken into account then a calculation on a single cell with a standard pseudopotential (i.e. without the core-hole) is enough. Since xspectra.x uses GIPAW reconstruction of the all electron wavefunction the pseudopotential needs to contain information about GIPAW reconstruction. There is no limit to the number of GIPAW projector that can be included. Note however that at least two projectors are needed to obtain XAS spectra converged up to 30-40 eV from the Fermi level. The use of a single projector is discouraged, particularly when semicore states are present. If more than two projectors are used, linear independence of the projectors should be explicitly verified (verbosity='high'). Once the scf charge density has been obtained, the xspectra.x code can be used as a post-processing tool. Note that the X-ray absorption spectra can be calculated on a larger mesh, different from that used in the PWscf scf run. Convergence need to be tested also for this second mesh. XSpectra calculates then the XAS electric dipole (for K and L edges) or electric quadrupole contributions (for K and L1 edges only), using the Lanczos method and the continued fraction. This approach does not require the explicit calculation of empty states and it is consequently very fast (only the charge density is needed). The code needs the radial core wavefunction of the initial core state in input. This wavefunction is included in the pseudo and can be extracted using the script upf2plotcore.sh in the directory ~/espresso/XSpectra/tools/ . Note that this script works only for UPF version 1. This is necessary to calculate the XAS matrix element. The output spectrum can be separated in its spin-up and spin-down polarizations. DFT+U calculations and collinear magnetism are possible. Ultrasoft pseudopotentials are allowed. Hybrid functionals not yet allowed. -------------------------------------------------------------------------- ======================================================================= NAMELIST / input_xspectra / calculation character (len=8) DEFAULT='' 'xanes_dipole', Perform dipolar calculation 'xanes_quadrupole', Perform quadrupolar calculation 'hpsi', Perform the test H*psi=E*Psi (debug option) edge character (len=16) DEFAULT='K', specifiy the edge to be calculated. 'K' specify the standard K-edge calculation 'L2' calculates the L2 edge, 'L3' calculates the L3 edge, 'L23' calculates both. However, it should be noted that in the single particle approximation the L3/L2 branching ration is exactly equal two 2. Thus a calculation of one of the edges is enough. lplus logical DEFAULT=.false. if lpus=.true. only transition 2p ---> d are allowed in the dipolar cross section for L23 edges. lminus logical DEFAULT=.false. if lminus=.true. only transition 2p ---> s are allowed in the dipolar cross section for L23 edges. prefix character (len=256) prefix of the pwscf output files outdir character (len=256) DEFAULT='./' directory tmp_dir or where the pwscf output files are stored verbosity character (len=4) DEFAULT='low' 'high',it checks linear dependence of PAW projectors and write details about the projectors. Note that GIPAW already perform a check on the linear dependence of the projectors even without this option. xiabs integer DEFAULT=1 type of the absorbing atom: position within the ATOMIC_SPECIES in pwscf input xkvec(1:3) real(DP) DEFAULT=(1.0,0.0,0.0) coordinates of the X-ray wave-vector k xepsilon(1:3) real(DP) DEFAULT=(0.0,0.0,1.0) coordinates of the incident X-ray polarization vector xcoordcrys logical DEFAULT=.true. .true. to use crystal coordinates for xkvec and xepsilon xonly_plot logical DEFAULT=.false. .false. the continued fraction is calculated for each k-point and at the end written on the save file .true. uses a previously calculated continued fraction (x_save_file) to re-plot the spectrum with different parameters (gamma broadening parameter,with occupied state,etc.) x_save_file character (len=256) DEFAULT=xanes.sav save file where results of the Lanczos calculation (a,b coefficients, etc.) are written If xonly_plot=.true., the x_save_file is read (read only) to get the Lanczos parameters calculated in a previous run Current version number is 2 ef_r (obsolete use xe0) The Fermi energy is determined from the SCF save directory. For an insulator, it is set to the energy of the highest occupied level. If the calculation is spin polarized, the largest of the Fermi energies corresponding to spin up and down is kept. If the zero of the spectrum needs to be changed, use xe0 (see below). xe0 real(DP) DEFAULT=1.d4 energy-zero for the spectrum in eV - must be set to the Fermi level if xonly_plot is .true. and the version of x_save_file is 1 (written with a previous version of the code). If x_save_file is 2 and xe0 is not specified then the zero energy of the spectrum is set at the Fermi level. - can also be used to set the zero energy for the calculation of the spectrum at an other value than the Fermi energy (for example, for an insulator, in the middle of the gap) xniter integer DEFAULT=2000 maximum number of iterations for Lanczos. The maximum number of iterations allowed must be lower than the number of vectors in the Hilbert space (i.e. the number of plane waves). xcheck_conv integer DEFAULT=5 number of iterations between 2 convergence tests: Xspectra checks convergency of the spectrum every xcheck_conv iterations of the Lanczos-basis construction. xerror real(DP) DEFAULT=0.01 convergence threshold for Lanczos calculation (eV) If the difference of two successive spectra (for a given k-point) is smaller than xerror, the convergence is achieved. show_status logical DEFAULT=.false. flag to show the status of the code U_projection_type character(len=16) DEFAULT='atomic' type of projection for DFT+U calculations (see the PWscf input file for more info) wf_collect logical DEFAULT=.false. must be true if wf_collect is enabled in the scf calculation time_limit integer DEFAULT=1.d8 time in seconds before stopping the calculation. If XSpectra stops because of the time limit, a and b coefficients of the incomplete continued fraction are stored in the .sav file. restart_mode character (len=32) DEFAULT='from_scratch' 'restart' if you want to restart from a .sav file where a and b coefficients of an incomplete continued fraction are stored. =============================================================================== NAMELIST / plot / xnepoint integer DEFAULT=100 number of energy points in the plot of the XAS spectrum xemax real(DP) DEFAULT=10.0 maximum energy in eV for the plot of the XAS spectrum xemin real(DP) DEFAULT=0.0 minimum energy in eV for the plot of the XAS spectrum cut_occ_states logical DEFAULT=.false. .false. the occupied states are visualized .true. the occupied states are smoothly cut from the plot terminator logical DEFAULT=.false. .true. to use the terminator function for the continued fraction .false. no terminator is used. gamma_mode character (len=256) DEFAULT='constant' 'constant': a constant broadening parameter (xgamma) is used for the XAS spectrum. 'variable': an energy-dependent broadening parameter is used: constant and equal to gamma_value(1) from xemin to gamma_energy(1), constant and equal to gamma_value(2) from gamma_energy(2) to xemax and linear from gamma_energy(1) to gamma_energy(2). 'file': the continued fraction uses an energy-dependent broadening parameter stored in file gamma_file. The broadening parameter (gamma of the continued fraction) is equivalent to the half width at half maximum of a Lorentzian used for a convolution xgamma real(DP) DEFAULT=0.1 constant broadening parameter to be used in the spectrum (eV) used for convergence and if gamma_mode='constant' gamma_energy(1:2) real(DP) energy values in eV of the 2 points of reference for variable gamma used if gamma_mode='variable' gamma_value(1:2) real(DP) gamma values in eV of the 2 points of reference used if gamma_mode='variable' gamma_file character (LEN=256) DEFAULT='gamma.dat' The file has to be formatted in two columns : energy1 gamma1 energy2 gamma2 where at energy1 the broadening parameter is gamma1. used if gamma_mode='file' ============================================================================== NAMELIST / pseudos / filecore, character (len=256) DEFAULT='Core.wfc' core wavefunction file r_paw(1:...) real(DP) DEFAULT=1.5*rc paw radii to be used in paw reconstruction. rpaw(1) corresponds to l quantum number=1 (electric dipole) rpaw(2) corresponds to l quantum number=2 (electric quadrupole) ============================================================================== In order to cut the occupied states, the program performs an integration over the variable t in ] 0, infinity [. For more details see ref. Ch. Brouder, M. Alouani, K. H. Bennemann, Phys. Rev. B 54 (1996) p.7334-49. The integration is done with t going in two opposite directions, from the start value cut_startt. So, the integration is done over ]cut_tinf,cut_startt] at least with step cut_stepl, and over [cut_startt,cut_tsup[ at least with step cut_stepu. There are two arrays of size cut_nmeml and cut_nmemu in order to save Green functions values. There is an area near the Fermi level of size cut_desmooth (in eV) where the cross section is interpolated in order to avoid a divergence. NAMELIST / cut_occ / cut_ierror real(DP) DEFAULT=1.d-7 convergence tolerance for one step in the integral cut_stepu real(DP) DEFAULT=1.d-2 integration initial step, upper side cut_stepl real(DP) DEFAULT=1.d-3 integration initial step, lower side cut_startt real(DP) DEFAULT=1.d0 integration start value of the t variable cut_tinf real(DP) DEFAULT=1.d-6 maximum value of the lower integration boundary cut_tsup real(DP) DEFAULT=100.d0 minimum value of the upper integration boundary cut_desmooth real(DP) DEFAULT=1.d-2 size of the interval near the fermi energy in which cross section is smoothed cut_nmemu integer DEFAULT=100000 size of the memory of the values of the green function, upper side cut_nmeml integer DEFAULT=100000 size of the memory of the values of the green function, lower side =================================================================================