mirror of https://gitlab.com/QEF/q-e.git
269 lines
11 KiB
Plaintext
269 lines
11 KiB
Plaintext
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XSPECTRA
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---------
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by C. Gougoussis, M. Calandra, A. Seitsonen and F. Mauri
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Please if you use the following program you should cite the
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following papers:
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C. Gougoussis, M. Calandra, A. Seitsonen, Ch. Brouder, A. Shukla, F. Mauri
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arXiv:0806.4706
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M. Taillefumier, D. Cabaret, A. M. Flank, and F. Mauri
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Phys. Rev. B 66, 195107 (2002)
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-----------------------------------------------------------------------
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XSpectra is a post-processing tools that relies on the output (the charge density)
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of the PWscf code (pw.x). Thus a scf calculation needs to be done before running
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xspectra.x.
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To simulate core-hole effects, a pseudopotential with a hole in the 1s
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state needs to be generated for the absorbing atom. Some of these
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pseudopotentials are available in the Xspectra examples directory.
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The self-consistent calculation is then performed on a supercell including
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the absorbing atom. The size of the supercell needs to be verified from
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system to system, since fairly large supercells are necessary for convergence.
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If core-hole effects need not to be taken into account then a calculation on
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a single cell with a standard pseudopotential (i.e. without the core-hole)
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is enough.
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Since xspectra.x uses GIPAW reconstruction of the all electron wavefunction
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the pseudopotential needs to contain informations about GIPAW reconstruction.
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There is no limit to the number of GIPAW projector that can be included. Note
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however that two projectors are typically enough to obtain XAS spectra
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converged up to 30-40 eV from the Fermi level.
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The use of a single projector is discouraged, particularly when semicore
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states are present.
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Once the scf charge density has been obtained the xspectra.x code can be
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used as a post-processing tool. Note that the X-ray absorption spectra
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can be calculated on a larger mesh, different from that used in the
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PWscf scf run. Covergence need to be tested also for this second mesh.
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Xspectra calculates then the XAS dipolar or quadrupolar contributions
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using the lanczos method and the continued fraction.
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This approach does not require the explicit calculation of empty states
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and it is consequently very fast (only the charge density is needed).
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The code needs the 1s radial core wavefunction (for the 1s state in the absence of a
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core-hole) in input. This is necessary to calculate the XAS matrix element.
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The output spectrum can be separated in its spin-up and spin-down polarizations.
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DFT+U calculations and collinear magnetism are possible.
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Ultrasoft pseudopotentials and non-collinear magnetism are currently
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under development.
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--------------------------------------------------------------------------
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=======================================================================
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NAMELIST / input_xspectra /
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calculation character (len=8) DEFAULT=''
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'xanes_dipole', Perform dipolar calculation
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'xanes_quadrupole', Perform quadrupolar
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calculation
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'fermi_level', calculate the Fermi level
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of the SCF run (xreadwf=.true.
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must be set in this case)
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'hpsi', Perform the test H*psi=E*Psi
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(debug option)
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prefix character (len=256)
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prefix of the pwscf output files
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outdir character (len=256) DEFAULT='./'
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directory tmp_dir or where the pwscf output
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files are stored
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verbosity character (len=4) DEFAULT='low'
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'high', it checks linear dependence of PAW
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projectors and write details about the
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projectors. Note that GIPAW already perform a
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check on the linear dependence of the
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projectors even without this option.
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xiabs integer DEFAULT=1
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type of the absorbing atom,
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(position in pwscf input)
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xkvec(1:3) real(DP) DEFAULT=(1.0,0.0,0.0)
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coordinates of the x-ray momentum k
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xepsilon(1:3) real(DP) DEFAULT=(1.0,0.0,0.0)
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coordinates of the incident x-ray
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polarization vector
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xcoordcrys logical DEFAULT=.true.
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.true. to use crystal coordinates for
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k and epsilon
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ef_r real(DP) DEFAULT=0.0
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Fermi energy in Rydberg. This value combined
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with the option cut_occupied_states can be
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used to exculde the occupiped states in a
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smooth way from the final plot.
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xonly_plot logical DEFAULT=.false.
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.false. the continued fraction is calculated
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for each k-point and at the end written
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on the save file
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.true. uses a previously calculated continued
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fraction (x_save_file) to replot the
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spectrum with different parameters
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(linewidth, different ef_r, etc. etc.)
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xread_wf logical DEFAULT=.false.
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.true. to read the wave functions of PWscf
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output
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x_save_file character (len=256) DEFAULT=xanes.sav
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save file where results of the Lanczos
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calculation are written (a,b vectors, etc. etc.).
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If xonly_plot=.true., the x_save_file is only
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read to get the a, b vectors and other lanczos
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parameters calculated in a previous run
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xniter integer DEFAULT=2000
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maximum number of iterations for lanczos.
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The maximum number of iterations allowed must
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be lower than the number of vectors in the
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Hilbert space (i.e. the number of plane waves)
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xcheck_conv integer DEFAULT=50
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number of iterations between 2 convergence
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tests
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show_status logical DEFAULT=.false.
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show the status of the code
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U_projection_type character(len=16) DEFAULT='atomic'
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type of projection for DFT+U calculations
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(see the PWscf input file for more info)
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wf_collect logical DEFAULT=.false.
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must be true if wf_collect is enabled
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in the scf calculation
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xerror real(DP) DEFAULT=0.01
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convergence threshold for lanczos
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calculation (eV)
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===============================================================================
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NAMELIST / plot /
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xnepoint integer DEFAULT=1000
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number of energy points in the plot of the
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XAS spectrum
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xgamma real(DP) DEFAULT=0.1
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linewidth to be used in the spectrum (eV)
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xemax real(DP) DEFAULT=10.0
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maximum energy in eV for the plot of the
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XAS spectrum
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xemin real(DP) DEFAULT=0.0
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minimum energy in eV for the plot of the
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XAS spectrum
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cut_occ_states logical DEFAULT=.false.
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.false. the occupied states are visualized
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.true. the occupied states are smoothly cut
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from the plot
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terminator logical DEFAULT=.false.
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.true. to use the terminator function for the
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continued fraction
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.false. no terminator is used.
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gamma_mode character (len=256) DEFAULT='constant'
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'constant' a constant linewidth is used for the
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XAS spectrum.
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'variable' a two step linewidth is used for the
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linewidth of the XAS spectrum. In this
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case the linewidth is constant and
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equal to gamma_value(1) from
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xemin to gamma_energy(1), constant and
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equal to gamma_value(2) from
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gamma_energy(2) to xemax and goes linearly
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from gamma_energy(1) to gamma_energy(2)
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'file' more exotical choices can be read from
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a file (gamma_file).
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gamma_file character (LEN=256) DEFAULT='gamma.dat'
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file used for nonconstant gamma, in case
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gamma_mode='file'. The file should be of the
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format energy1 gamma1
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energy2 gamma2
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where at energy1 the linewidth is gamma1.
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Then the values are connected by lines.
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gamma_energy(1:2) real(DP)
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energy values of the 2 points of reference for variable gamma
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in case gamma_mode='variable'
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gamma_value(1:2) real(DP)
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gamma values of the 2 points of reference for variable gamma
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in case gamma_mode='variable'
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==============================================================================
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NAMELIST / pseudos /
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filecore, character (len=256)
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core wavefunction file
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r_paw(1:...) real(DP) DEFAULT=1.5*rc
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paw radii to be used in paw reconstruction.
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A good choice to avoid linearly dependent
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projectors is 3*r_pseudo/2 or grater.
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==============================================================================
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In order to cut the occupied states, the program performs an integration
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over the variable t in ] 0, infinity [.
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See ref. Ch. Brouder, M. Alouani, K. H. Bennemann, Phys. Rev. B 54 (1996) p.7334-49.
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The integration is done with t going in two opposite directions,
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from the start value cut_startt. So, the integration
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is done over ]cut_tinf,cut_startt] at least with step cut_stepl, and
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over [cut_startt,cut_tsup[ at least with step cut_stepu. There are two arrays of size
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cut_nmeml and cut_nmemu in order to save green functions values. There is an area near
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the fermi level f size cut_desmooth (in eV) where the cross section is interpolated
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in order to avoid a divergence.
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NAMELIST / cut_occ /
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cut_ierror real(DP) DEFAULT=1.d-7
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convergence tolerance for one step in the integral
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cut_stepu real(DP) DEFAULT=1.d-2
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integration initial step, upper side
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cut_stepl real(DP) DEFAULT=1.d-3
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integration initial step, lower side
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cut_startt real(DP) DEFAULT=1.d0
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integration start value of the t variable
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cut_tinf real(DP) DEFAULT=1.d-6
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maximum value of the lower integration boundary
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cut_tsup real(DP) DEFAULT=100.d0
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minimum value of the upper integration boudary
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cut_desmooth real(DP) DEFAULT=1.d-2
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size of the interval near the fermi energy
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in which cross section is smoothed
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cut_nmemu integer DEFAULT=100000
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size of the memory of the values of the
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green function, upper side
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cut_nmeml integer DEFAULT=100000
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size of the memory of the values of the
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green function, lower side
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=================================================================================
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