First, we create a directory for the BSE calculations. All subsequent calculations with the exciting code will be performed in that directory
mkdir lif-exciting
cd lif-exciting/
First, we need to calculate the DFT electronic structure and F K edge absorption spectrum. We do this with the following input file:
<input>
<title>LiF F K Edge</title>
<structure speciespath="./">
<crystal scale="1.8897543768634038">
<basevect> 0.00000000 2.01297060 2.01297060</basevect>
<basevect> 2.01297060 0.00000000 2.01297060</basevect>
<basevect> 2.01297060 2.01297060 0.00000000</basevect>
</crystal>
<species speciesfile="Li.xml">
<atom coord=" 0.00000000 0.00000000 0.00000000" />
</species>
<species speciesfile="F.xml">
<atom coord=" 0.50000000 0.50000000 0.50000000" />
</species>
</structure>
<groundstate
do="fromscratch"
ngridk="2 2 2"
xctype="GGA_PBE_SOL"/>
<xs
xstype="BSE"
ngridk="2 2 2"
ngridq="2 2 2"
rgkmax="4.0"
vkloff="0.05 0.15 0.25"
nempty="30"
gqmax="2.0"
broad="0.0276"
tappinfo="true"
tevout="true">
<energywindow
intv="23.6 24.6"
points="1500" />
<screening
screentype="full"
nempty="100"/>
<BSE
chibar0="false"
chibar0comp="1"
xas="true"
xasspecies="2"
xasatom="1"
xasedge="K"
bsetype="singlet"
nstlxas="1 2"/>
<qpointset>
<qpoint>0.0 0.0 0.0</qpoint>
</qpointset>
<storeexcitons MinNumberExcitons="1" MaxNumberExcitons="32"/>
</xs>
</input>We see that we write all 32 excitonic eigenstates to file. In a converged BSE calculation, the number of eigenstates is huge and one might need to consider carefully how many are necessary for a converged RIXS spectrum. Generally, this will depend on the range of excitation energies one is interested in. The higher the excitation energy is beyond the absorption edge, the more eigenstates are required.
Executing the exciting code with this input.xml will generate a output file named bse_output.h5 which we rename by
mv bse_output.h5 core_output.h5
Renaming is important, because otherwise it will be overwritten when we calculate the valence excitation spectrum.
An important ingredient of the RIXS calculation are the momentum matrix elements between the core and conduction states. These can be calculated with low computational cost by adjusting the input files such that it looks like:
<input>
<title>LiF F K Edge</title>
<structure speciespath="./">
<crystal scale="1.8897543768634038">
<basevect> 0.00000000 2.01297060 2.01297060</basevect>
<basevect> 2.01297060 0.00000000 2.01297060</basevect>
<basevect> 2.01297060 2.01297060 0.00000000</basevect>
</crystal>
<species speciesfile="Li.xml">
<atom coord=" 0.00000000 0.00000000 0.00000000" />
</species>
<species speciesfile="F.xml">
<atom coord=" 0.50000000 0.50000000 0.50000000" />
</species>
</structure>
<groundstate
do="fromscratch"
ngridk="2 2 2"
rgkmax="4.0"
xctype="GGA_PBE_SOL"/>
<xs
xstype="BSE"
ngridk="2 2 2"
ngridq="2 2 2"
rgkmax="4.0"
vkloff="0.05 0.15 0.25"
nempty="30"
gqmax="2.0"
broad="0.0276"
tappinfo="true"
tevout="true">
<energywindow
intv="23.6 24.6"
points="1500" />
<screening
screentype="full"
nempty="100"/>
<BSE
chibar0="false"
chibar0comp="1"
xas="true"
xasspecies="2"
xasatom="1"
xasedge="K"
bsetype="singlet"
nstlxas="1 2"
distribute="true"/>
<qpointset>
<qpoint>0.0 0.0 0.0</qpoint>
</qpointset>
<plan>
<doonly task="writepmatxs"/>
<doonly task="writepmatasc"/>
</plan>
</xs>
</input>The input file is nearly identical to the one used for the calculation of the F K edge,
with the only change being the subelement plan which triggers the calculation
of the momentum matrix elements (writepmatxs) and the output to HDF5 file
(writepmatasc). The matrix elements are written to the file bse_output.h5.
We rename the file again
mv bse_output.h5 pmat.h5
Following the F K edge spectrum, we now calculate the valence excitation
spectrum. While many parameters (such as nstlbse and gqmax) can be different
between the two BSE calculations, the parameters ngridk, ngridq, and
vkloff HAVE TO BE IDENTICAL. To make sure that the wavefunctions are
identical in both calculations, we do not calculate the DFT electronic structure
again (a repeated diagonalization of the Kohn-Sham Hamiltonian can introduce a
different global phase, which alters the results). As such, the input file looks
like this
<input>
<title>LiF Valence Spectrum</title>
<structure speciespath="./">
<crystal scale="1.8897543768634038">
<basevect> 0.00000000 2.01297060 2.01297060</basevect>
<basevect> 2.01297060 0.00000000 2.01297060</basevect>
<basevect> 2.01297060 2.01297060 0.00000000</basevect>
</crystal>
<species speciesfile="Li.xml">
<atom coord=" 0.00000000 0.00000000 0.00000000" />
</species>
<species speciesfile="F.xml">
<atom coord=" 0.50000000 0.50000000 0.50000000" />
</species>
</structure>
<groundstate
do="fromscratch"
ngridk="2 2 2"
rgkmax="4.0"
xctype="GGA_PBE_SOL"/>
<xs
xstype="BSE"
ngridk="2 2 2"
ngridq="2 2 2"
rgkmax="4.0"
vkloff="0.05 0.15 0.25"
nempty="30"
gqmax="2.0"
broad="0.0276"
tappinfo="true"
tevout="true">
<energywindow
intv="0 1.0"
points="1500" />
<screening
screentype="full"
nempty="100" />
<BSE
chibar0="false"
chibar0comp="1"
bsetype="singlet"
nstlbse="2 5 1 2"
distribute="true" />
<qpointset>
<qpoint>0.0 0.0 0.0</qpoint>
</qpointset>
<storeexcitons MinNumberExcitons="1" MaxNumberExcitons="64"/>
<plan>
<doonly task="writepmatxs"/>
<doonly task="scrcoulint"/>
<doonly task="exccoulint"/>
<doonly task="bse"/>
</plan>
</xs>
</input>
Once again, we write all 64 eigenstates of the BSE to file. With the plan
subelement, we specify which tasks need to be performed. We re-calculate neither
the KS electronic structure nor the screening. These quantities are supossed to
be identical in the two BSE calculations. Finally, we also rename this file
mv bse_output.h5 optical_output.h5
From this point on, only the files core_output.h5, pmat.h5, and optical_output.h5 are required. We will move them to a different directory
mkdir ../lif-rixs
mv *.h5 ../lif-rixs
cd ../lif-rixs
Now we have to create the file input.cfg that describes the input parameters for the BRIXS calculation. The file contains the following:
omega= 653 654 655 656
pol=1.0 0.0 0.0
broad=0.15
nblocks=2
eigstates_optical=64
eigstates_core=32
Here, we specify the excitation energies (omega) and the polarization (pol)
of the incoming photon. Note that the polarization should be identical to the
one specified by the parameter chibar0comp in the exciting calculations.
We furthermore specify the lifetime broadening (broad) of the intermediate
state. With eigstates_optical and eigstates_core we specify how many
eigenstates of the corresponding BSE calculations are used. Finally, nblocks
defines in how many blocks the eigenvectors should be separated. More blocks
will reduce the memory requirements of the calculations, but might increase the
runtime. A good estimate is to use the number of k-points along a specific
direction.
We then execute the first BRIXS executable with
/path/to/BRIXS/bin/rixs-pathway-serial
which creates the file data.h5. The final result is then obtained by executing
/path/to/BRIXS/bin/rixs-oscstr-serial
The final result is stored in the file rixs.h5.