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XSPH: Cross-section and phase shifts


Relativistic dipole matrix elements (alpha form) are calculated using atomic core and normalized continuum wavefunctions. Polarization dependence is optionally incorporated in the dipole-operator. Scattering phase shifts are determined by matching at the muffin-tin radius. Additionally, tex2html_wrap_inline3197-projected density of states can be calculated in this module, but it is of limited quality due to finite cluster calculations and neglect of nonspherical corrections.

 ELLIPTICITY    ellipticity x y z    (Useful)

This card is used with the POLARIZATION card (see below). The ellipticity is the ratio of amplitudes of electric field in the two orthogonal directions of elliptically polarized light. Only the absolute value of the ratio is important for nonmagnetic materials. The present code cannot distinguish left and right circular polarization. A zero value of the ellipticity corresponds to linear polarization, and unity to circular polarization. The default value is zero.

x, y, z are coordinates of any nonzero vector in the direction of incident beam. This vector should be approximately normal to the polarization vector.
* circular polarization about the z-axis
  ELLIPTICITY  1.0  0.0  0.0  -2.0

 POLARIZATION    x y z    (Useful)

This card specifies the direction of the electric field of the incident beam or the main axis of the ellipse in the case of elliptical polarization. x, y, z are the coordinates of the nonzero polarization vector. The ELLIPTICITY card is not needed for linear polarization. If the POLARIZATION card is omitted, spherically averaged xafs will be calculated.

Note that polarization reduces the degeneracy of the paths, increasing the calculation time. Choosing polarization in the directions of symmetry axes will result in a faster calculation.
POLARIZATION  1.0  2.5  0.0

 EXAFS    [xkmax]    (Standard)

The EXAFS card is used to change the maximum value of k for EXAFS calculations. Default value is 20 Åtex2html_wrap_inline3279.
EXAFS 17.5

 LDOS    emin emax eimag    (Useful)

The angular momentum projected density of states is placed by default on a standard grid currently fixed at 84 points. emin and emax are the minimum and maximum energies of the tex2html_wrap_inline3197DOS calculation and eimag is the imaginary part of potential used in the calculations. This is equivalent to Lorentzian broadening of the tex2html_wrap_inline3197DOS with half-width = eimag. If eimag is negative, the code automatically sets it to be 1/3 of the energy step. The output will be written again into 'rholNN.dat' files. To obtain LDOS you must run the second module by setting the second CONTROL argument to 1. If 84 points are not enough, you can divide the energy range by 2 and run the code twice. The LDOS card is very useful when examining densities of states for interpreting XANES or when the self-consistency loop fails or gives very strange results. For crystals our LDOS will always be broadened due to the effect of finite cluster size.
*       emin emax eimag
  LDOS  -20  20   0.2

 XANES    [xkmax xkstep estep]    (Standard)

The XANES card is used when a calculation of the near edge structure including the atomic background and absolute energies are desired.

The XANES calculation is currently limited to the (extended) continuum spectrum beyond the Fermi level. Thus bound states are not generally included; however, in molecules weakly bound states that are below the vacuum but above the muffin-tin zero will show up as resonances. The absolute energies are based on atomic total energy calculations using the Dirac-Fock-Desclaux atom code. The accuracy of this approximation varies from a few eV at low Z to a few hundred eV for very large Z. All parameters are optional. Default: XANES not calculated unless card is present.

The optional parameters are used to change the output energy mesh for the XANES calculation. xkstep specifies the size of the output k grid far from the edge. xkmax is the maximum k value of the XANES calculation. FMS calculations are are not accurate beyond about k=6; for larger values of k, e.g. k=20 with the path expansion, FMS must be turned off. The grid at the edge will be regular in energy with a step size of estep. The default values are tex2html_wrap_inline3295, tex2html_wrap_inline3297, and tex2html_wrap_inline3299, where vi0 is given by the EXCHANGE card described in Section 2.3.

* finer grid for XANES calculation
XANES  6. .05 .3

 RPHASES        (Advanced)

If this card is present, real phase shifts rather than complex phase shifts will be used. The results of the calculation will not be accurate. This option is intended to allow users to obtain real scattering phase shifts for use with other programs, or for diagnostic purposes. The phase shifts can be written to output files 'phaseNN.dat' using the PRINT card. If the RPHASES card is present, these will be the real phase shifts.

next up previous contents
Next: FMS: Full multiple scattering Up: Input File Controls Previous: POT: Scattering potentials
John J. Rehr
Thu Jul 1 14:31:17 PDT 1999