Core Loss EELS

See examples/Si2_CORE/. This is a simple example of using optados for calculating core level absorption spectra for crystalline silicon. We assume that the reader is familiar with the previous section on calculating DOS.


  • We begin by running a castep calculation using the files provided in examples/Si2_CORE. Note that we specify a pseudopotential file for one atom, and an on-the-fly pseudopotential for the other atom. This looks a bit weird! It is simply a way to only compute the EELS for one atomic site (core-loss spectra can only be computed for atoms described by on-the-fly potentials)
  • Execute optados using the optados input file provided and the file Si2_CORE_core_edge.dat will be created. The file contains two columns, the first is the energy and the second is the spectrum. This file contains the following edges:
 # Si 1    K1
 # Si 1    L1
 # Si 1  L2,3
 # Si 2    K1
 # Si 2    L1
 # Si 2  L2,3

i.e. all edges from the atom are produced.

  • Optados has also written a grace file

xmgrace Si2_CORE_core_edge.agr

This spectrum contains lots of fine detail. To compare with experiment we can include lifetime and instrument broadening effects. First let's add some Gaussian broadening to simulate instrument effects. Add the following the odi file and re-run.


To compare the broadened and un-broadened spectra

xmgrace Si2_CORE_core_edge_broad.agr Si2_CORE_core_edge.agr

Now add some lifetime broadening (look up the meaning of the keywords in the optados user guide


Including a Core hole

To include a core-hole in the calculation, first one atom is chosen to have the excitation. To begin, we will keep two atoms in the unit cell and distinguish one atom by changing the

Si:1  0.0     0.0     0.0
Si:2 0.25    0.25    0.25

Si:1 2|1.8|1.8|1.3|2|3|4|30:31:32LGG{1s1.00}(qc=4) ! 1s core hole
Si:2 Si_STD_OTF.usp

in the Si2_CORE.cell file. The atom Si:1 is the one to have a core-hole. To create a core-hole we remove a 1s electrons from the electronic configuration used in the generation of the pseudopotential. Information about the pseudopotentials is included at the top of the Si2_CORE.castep file.

 | Pseudopotential Report-Date of generation 16-05-2012     |
 | Element: Si Ionic charge:  4.00 Level of theory: LDA     |
 |                                                          |
 |               Reference Electronic Structure             |
 |         Orbital         Occupation         Energy        |
 |            3s              2.000           -0.400        |
 |            3p              2.000           -0.153        |
 |                                                          |
 |                 Pseudopotential Definition               |
 |        Beta     l      e      Rc     scheme   norm       |
 |          1      0   -0.400   1.797     qc      0         |
 |          2      0    0.250   1.797     qc      0         |
 |          3      1   -0.153   1.797     qc      0         |
 |          4      1    0.250   1.797     qc      0         |
 |         loc     2    0.000   1.797     pn      0         |
 |                                                          |
 | Augmentation charge Rinner = 1.298                       |
 | Partial core correction Rc = 1.298                       |
 | "2|1.8|1.8|1.3|2|3|4|30:31:32LGG(qc=4)"                  |
 |      Author: Chris J. Pickard, Cambridge University      |

The line


specifies the parameters used to create the pseudopotential. We use this as the starting point and then remove one of the core 1s electrons to create a core-hole pseudopotential. This is done by including {1s1.00} in the pseudopotential string as shown:


If, instead of removing a 1s electron, we wanted to remove a 2s electron from the core, we would have included {2s1.00} instead of {1s1.00} in the pseudopotential string.

To maintain the neutrality of the cell, we include

  CHARGE : +1 

in the Si2_CORE.param file. Run the calculation. Compare the K-edge from the core-hole calculation with the previous non-core-hole calculation.

  • The periodic images of the core-hole will interact with one another. As this is unphysical, we need to increase the distance between the core-holes. This is done by creating a supercell. To start with we use a face-centred unit cell rather than the primitive unit cell. This is done by changing the lattice parameters and fractional co-ordinates to:
            5.46  0.00 0.00
            0.00  5.46 0.00
            0.00  0.00 5.46

     Si:1      0.0000000000    0.0000000000    0.0000000000
     Si:2      0.5000000000    0.5000000000    0.0000000000  
     Si:2      0.5000000000    0.0000000000    0.5000000000
     Si:2      0.0000000000    0.5000000000    0.5000000000
     Si:2      0.2500000000    0.2500000000    0.2500000000
     Si:2      0.7500000000    0.2500000000    0.7500000000
     Si:2      0.2500000000    0.7500000000    0.7500000000
     Si:2      0.7500000000    0.7500000000    0.2500000000

Run optados and compare the spectrum from the face-centred unit cell with that from the primitive unit cell. Continue constructing larger unit cells until the core-hole spectrum stops changing with increasing separation between the periodic images.

Other things to try include

  1. Changing the geometry from polycrystalline to polarised
  2. Repeat for Graphite (or graphene)