XANES in Diamond: Difference between revisions

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=== {{FILE|POSCAR}} ===
=== {{FILE|POSCAR}} ===
cubic diamond
cubic diamond
3.567
  3.567
  0.5 0.5 0.0
  0.5 0.5 0.0
  0.0 0.5 0.5
  0.0 0.5 0.5
  0.5 0.0 0.5
  0.5 0.0 0.5
2
  2
direct
direct
  0.0 0.0 0.0
  0.0 0.0 0.0
  0.25 0.25 0.25
  0.25 0.25 0.25

Revision as of 15:22, 12 April 2018

Task

Calculation of the XANES K-edge in diamond using the supercell core-hole method.

Input

POSCAR

cubic diamond
 3.567
0.5 0.5 0.0
0.0 0.5 0.5
0.5 0.0 0.5
 2
direct
0.0 0.0 0.0
0.25 0.25 0.25
  • We will not use this structure as input but rather use it to construct a super cell from it that is actually used in the calculations.

INCAR

System = DIAMOND
ALGO = FAST
ISMEAR = 0; SIGMA = 0.1;
ICORELEVEL = 2
CLNT = 1
CLN = 1
CLL = 0
CLZ = 1.0
CH_LSPEC = .TRUE.
CH_SIGMA = 0.5
NBANDS = 300
LREAL = A
  • To promote a core electron into the conduction bands and hence create the core-hole ICORELEVEL=2 has to be set. This corresponds to the final state approximation
  • CLNT=1 selects the first atom species in the POSCAR file.
  • CLN=1 selects main quantum number 1 (hence K-edge).
  • CLL=0 selects angular quantum number 0 (s).
  • CLZ=1.0 selects the charge of the core hole. By setting this number to a fractional value we can mimick different screening of the electrons. Since this purely exploits error cancellation and the physical background of non-integer charges is not defined well, it should be only used with caution.
  • By setting CH_LSPEC=.TRUE. we enable the calculation of matrix elements between core and conduction states and the calculation of the core electron absorption spectrum.
  • The broadening of the core electron absorption spectrum is controlled by the tag CH_SIGMA. Usually it is good practice to set this value low and broaden the spectrum in post processing.
  • We have to set NBANDS to a larger value to consider enough conduction band states in the calculation.
  • Since super cells are used the calculation of the projection operators in real space is much faster, hence LREAL=A is set.

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