Calculations with SPEX
Note: This tutorial of the Fleur Workshop 2021 is based on version 05.07 of the Spex code.
1. GW calculations
The GW selfenergy is an approximation to the electronic selfenergy used in manybody perturbation theory. This theory allows excited state properties to be calculated on the basis of a meanfield system, which is usually taken from a previous DFT calculation, e.g., using the localdensity approximation (LDA). One says that LDA is the starting point for the GW calculation.
Thus, in order to carry out a GW calculation, we first have to perform a selfconsistent DFT calculation with the FLEUR code. For the latter, you will use your compiled FLEUR program.
1.1 Installation
As a first step, we are going to install the SPEX code. Copy the file "spex05.07.tgz" to your working directory and unpack it:
cp spex05.07.tgz workdir
tar xvzf spex05.07.tgz
Enter the directory spex05.07
:
cd spex05.07
In order to configure the compilation for your system (choice and test of compiler, search for libraries, etc.), you have to run the configure script. The script allows for customization through command line arguments:
./configure withwan prefix=$HOME withhdf5=/usr/lib/x86_64linuxgnu/hdf5/serial/
In your docker image judft/fleur:2021
, the HDF5 library is not in a standard directory. Therefore, we have to specify the HDF5 root directory with the option withhdf5
.
A short explanation of the options:
withwan
: Compile with Wannier support (searches for and links in the Wannier90 library),withhdf5=PATH
: Specify HDF5 root directory,prefix=PREF
: Install executables (and shell scripts) into PREF/bin (defaults to PREF=/usr/local; we choose PREF to be your home directory).enablempi
: Compile parallel version using MPI 3.1 (not needed for the tutorial),withdft=fleurR6
: Compile the interface for the new MaX Release 6 of FLEUR (this is the default; therefore, not needed),
We do not have ScaLAPACK in the prepared Docker image. Therefore, we cannot compile an MPIparallelized version of SPEX.
Compile the source! This will take a few minutes. At the end, you will be asked to install the binaries.
make
For faster compilation in parallel, use make j
.
You have just compiled the executables:
spex.inv
: For systems with inversion symmetry (and without SOC)spex.noinv
: For systems without inversion symmetry (or with SOC)spex.extr
: Extraction utility
These executables can be installed by
make install
Together with the aforementioned executables, we have also installed the following shell scripts
spex
: SPEX launcher scriptspex.band
: For bandstructure calculationsspex.selfc
: For selfconsistent calculations (e.g., QSGW)
The reason for having two executables, one for systems with inversion symmetry (spex.inv
) and the other for systems without (spex.noinv
),
is that many quantities can be declared as realvalued instead of complexvalued arrays in the case of inversion symmetry,
which speeds up the computation considerably.
The executables (spex.inv
or spex.noinv
) can be called directly. Alternatively, the launcher script allows simply running spex
in both cases. You can choose one of the two options. In the following, we will use spex
for simplicity (as in the second option).
The SPEX manual is here. There is also a PDF version of the manual: "spex05.07/docs/spex.pdf".
1.2 Silicon
1.2.1 DFT as the meanfield starting point
Your first GW calculation will be on bulk Silicon. As a first step, we must perform a DFT calculation that creates the necessary input for GW. Silicon crystallizes in a diamond lattice (fcc lattice with a twoatom basis) with a lattice constant of 5.43 Å (10.26 Bohr).
You can create your own FLEUR input file "inp_Si" as you have already learned it or use the prepared one "Si/inp_Si". If you create it yourself, it should contain the line &expert spex=1 /
, which tells FLEUR to create some additional files (containing structural information etc.) for the subsequent GW calculation.
We create the Fleur input file "inp.xml" by running "inpgen":
inpgen f inp_Si
Then, perform a DFT calculation for Silicon as usual, which produces a selfconsistent density and effective potential. Make sure that the calculation has converged.
1.2.2 Special kpoint set
As the GW selfenergy is nonlocal, its evaluation involves summations over two kpoints, k and q, and their sum k+q. Thus, we must make sure that the set of kpoints forms an equidistant regular mesh, which contains k+q as an element if k and q are elements of the set. Furthermore, the Γ point (k=0) must be included. (In other words, the kpoint set must form a group under vector addition.) The explicit inclusion of the Γ point is necessary because of the divergence of the Coulomb interaction in the longwavelength limit.
We can now proceed in two ways:
 SPEX creates the new kpoint set (Section 1.2.3). FLEUR runs a oneshot calculation on that new kpoint set (Section 1.2.4). Finally run the GW calculation with SPEX (Section 1.2.5).
 Directly run the GW calculation with SPEX (Section 1.2.5) with an additional line in the input file "spex.inp":
ITERATE
.
Obviously, the second procedure is much simpler as it spares us two runs (with SPEX and FLEUR). The first procedure is more flexible, though. It is required,
for example, for selfconsistent calculations (QSGW etc.). We leave it up to you, which procedure you choose. You might want to try both. Starting from Sec. 1.2.6, we assume that ITERATE
is used for simplicity.
1.2.3 Create new kpoint set
The new kpoint set can be created in the following way: Create an input file for SPEX, named "spex.inp", with only one entry:
BZ 4 4 4
The entry specifies a 4×4×4 kpoint set. Then, run SPEX with the argument w
spex w
This creates a new kpoint set in "kpts.xml" named "spex".
1.2.4 Run FLEUR on new kpoint set
Now we let FLEUR generate the wavefunctions and energies on this new kpoint set.
Important: This should be a oneshot calculation without selfconsistency. For this purpose, we must modify the FLEUR "inp.xml" file in the following way:

Set the
spex
flag to "2" (line 13):
<expertModes spex="2" secvar="F"/>

Set
numbands
to "180" (line 7):
<cutoffs Kmax="3.60000000" Gmax="11.00000000" GmaxXC="9.20000000" numbands="180"/>

Set
listName
to "spex" (line 19):<kPointListSelection listName="spex"/>
Setting spex="2"
lets FLEUR create additional files that are necessary for the GW calculation.
Setting numbands="180"
sets the number of bands (here, 180) returned by the diagonalization routine of FLEUR. This number must comprise occupied states
and a number of unoccupied states, as well, because the GW scheme involves sums over unoccupied states. (The default numbands="0"
would let FLEUR
generate only the occupied states.)
Setting listName="spex"
tells FLEUR to use the kpoint set that we have just created in the previous step.
After these changes have been applied to "inp.xml", you can run FLEUR again. This creates the necessary input data for SPEX. Note that you do not have to repeat the
steps described in Section 1.2.3 and 1.2.4 for each SPEX run unless you change the kpoint set (BZ
) or you add special additional kpoints (KPT +=...
), see below.
1.2.5 Perform the GW calculation
We have already created the SPEX input file "spex.inp" above. For the GW calculation, we must add a few parameters now. Change your "spex.inp" to look like this:
BZ 4 4 4
KPT X=1/2*(0,1,1) L=1/2*(0,0,1)
JOB GW 1:(1,2,5) X:(1,3,5) L:(13,5)
NBAND 80
Note: If you have jumped here from Section 1.2.2 (without performing the oneshot FLEUR calculation), you have to add the line
ITERATE
to the input file, which tells SPEX to first prepare the eigensolutions before running the GW calculation. For simplicity, we will from now on use the keyword ITERATE
.
A few remarks about the input file: The order of keywords (BZ
, KPT
, ...) is not important. A keyword must not be given more than once. Empty lines are allowed. Everything after #
is interpreted as a comment (and ignored).
The second line of the above input file defines some kpoint labels, here for the X and L point. These labels are used in the job definition (third line):
GW
stands for the GW approximation; then the states (on which the quasiparticle correction is to be applied) are defined
with the syntax "kpoint:(band indices)". The kpoint can be specified by its index (see list of kpoints in the output, e.g, "1" for the Γ point)
or by a kpoint label.
We have chosen to define the band indices in the job definition in such a way that, in the case of degeneracy, only one state out of a group of degenerate states is
calculated. For example, the second, third, and fourth state at Γ (kpoint index 1) are degenerate. You can also include all states with 1:(15)
.
The keyword NBAND
defines the number of bands (occupied and unoccupied) used in the GW calculation. Obviously, this number must not exceed the available number
of bands, which corresponds either to the numbands
parameter of "inp.xml" (180) or, if you use ITERATE
, to the number of LAPW basis functions.
If you omit the NBAND
definition, SPEX takes into account all available states.
Now all necessary files are created, and we can perform the GW calculation with SPEX. When SPEX is run, it writes information to the two standard streams of the shell: standard output (stdout) and standard error (stderr). The first stream is used for the normal output, the second for warning and error messages. Normally, both streams are directed to the screen, but they can also be directed to a file using suitable shell commands. Here are some examples:
spex
: Both output and error streams are written to the screen (not recommended).spex > spex.out
: The output stream is written to the file "spex.out", which you can later open with your favourite editor. The error stream is directed to the screen.spex &> spex.out
: Both streams are written to the file "spex.out".spex  tee spex.out
: The output stream is written to the screen but also to the file "spex.out" (recommended). This allows you to monitor the calculation while it is running. The error stream is written to the screen.spex 2> spex.err  tee spex.out
: The output stream is written to the terminal, but also to the file "spex.out". The error stream is written to the file "spex.err".
You should increase the horizontal size of your terminal a little bit because some lines of the output exceed the standard 80 characters of the terminal.
1.2.6 A longer input file
Before we investigate the output file, let us have a look at the input file again. The one used in the previous section was a minimal one. A typical GW calculation requires a quite large number of parameters. When these parameters are not given in the input file (as above), SPEX uses default parameters or determines reasonable ones from the DFT data. Of course, all parameters can also be defined explicitly in the input file. A longer input file for the above calculation could look like this:
# silicon bulk
BZ 4 4 4
KPT X=1/2*(0,1,1) L=1/2*(0,0,1)
JOB GW 1:(1,2,5) X:(1,3,5) L:(13,5)
NBAND 80
ITERATE
SECTION MBASIS
LCUT 4
GCUT 3.6
SELECT 2;3
OPTIMIZE MB 4.0
END
SECTION SUSCEP
HILBERT 50 30
END
SECTION SENERGY
MESH 10+2 10.0
CONTINUE
END
Note: The empty lines and the twocharacter indentation in the sections are used only for clarity. Everything after #
is interpreted as a comment.
Some explanations:
The section MBASIS
contains the definition of the mixed product basis. The keyword HILBERT
defines the realfrequency mesh for the Hilbert transformation
of the susceptibility (polarization function).
The section SENERGY
defines parameters for the evaluation of the selfenergy. The keyword MESH
defines the imaginaryfrequency mesh.
The keyword CONTINUE
tells SPEX to use analytic continuation (PadÃ© extrapolation) of the selfenergy from the imaginary to the realfrequency axis.
(This is the default.)
1.2.7 Investigating the results in "spex.out"
Have a look at the output file "spex.out". After reading and checking all parameters (routines "getinput" and "checkinput") and setting up the mixed product basis (routine "mixedbasis"), SPEX calculates the Coulomb matrix (routines "structureconstant" and "coulombmatrix"). Then, SPEX evaluates the exchange selfenergy (routines "exchange_core" and "exchange"), which corresponds to a oneshot HartreeFock (HF) calculation, and the HF energies are printed. Then, the correlation selfenergy is constructed in a kpoint loop, which consumes most of the CPU time. For each kpoint the susceptibility (or polarization function P) is calculated (routine "susceptibility"), from which the screened interaction W=(1vP)^{1} is obtained, and the correlation selfenergy Σ=iG(Wv) is updated. Finally, the selfenergy is analytically continued to the realfrequency axis and the quasiparticle equation is solved. The results are written in a table:
Bd vxc sigmax sigmac Z KS HF GW lin/dir
1 12.11172 19.10561 6.57060 0.64929 6.20997 13.20387 6.37436 0.72185
1.04269 0.10593 6.37825 0.71237
2 13.21577 14.83300 1.14256 0.76185 5.60804 3.99081 5.24641 0.00147
0.00088 0.00169 5.24744 0.00162
5 11.42949 6.88796 4.16444 0.75863 8.16243 12.70396 8.44847 0.00690
0.00667 0.00489 8.44752 0.00710
The columns are
Bd
: the band index,vxc
: expectation value of v_{xc}, e.g., 12.11172 eV,sigmax
: expectation value of the exchange selfenergy Σ_{x}, e.g., 19.10561 eV,sigmac
: expectation value of the correlation selfenergy Σ_{c}, e.g., (6.57060+1.04269i) eV (note that the selfenergy is complex),Z
: renormalization factor Z, e.g., (0.649290.10593i) eV,KS
: KohnSham value (from the previous DFT calculation), e.g., 6.20997 eV,HF
: HartreeFock value (oneshot), e.g., 13.20387 eV,GW
: GW quasiparticle values, e.g., (6.37436+0.72185i) eV (solution of a linearized, i.e., approximated, quasiparticle equation) and (6.37852+0.71237i) eV (exact solution of the nonlinear quasiparticle equation). Usually, there is only little difference between the two values.
Look up the KohnSham, HF, and GW direct and indirect gaps (i.e., Γ→Γ and Γ→X) and compare with the experimental values,
3.4 eV and 1.17 eV, respectively. Note that the conduction band minimum is not exactly at the X point, though (see below).
The fourth band is the highest valence band: There are two silicon atoms per unit cell. Each contributes four electrons. So, there are eight electrons in the
unit cell. (You can find this line in the output: Number of valence electrons: 8
.)
Then, dividing by two (two electron spins) gives four.
1.2.8 Contour integration
Instead of analytic continuation, you can use contour integration for evaluating the selfenergy. This is more accurate, but takes more CPU time.
Simply define CONTOUR 0.01
in the file "spex.inp" inside a section SENERGY
:
SECTION SENERGY
CONTOUR 0.01
END
(or replace CONTINUE
by CONTOUR 0.01
if you are using the input file of Section 1.2.6). Then rerun the calculation.
(You might want to use a different output file.) The argument "0.01" tells SPEX to calculate the selfenergy at two energies symmetrically around
each KS energy ε (ε0.01 htr and ε+0.01 htr), which enables a linearization of the selfenergy.
Therefore, we only get the "linearized" quasiparticle result.
(We can also obtain both solutions again by specifying a range of values instead of the single argument, e.g., CONTOUR +{0.05:0.05,0.01}
.)
Compare the GW results with the previous calculation.
1.2.9 Band convergence
You can play around with the parameters, e.g., increase the number of bands (keyword NBAND
) to 100 and 120 and check the bandgap convergence.
You can also experiment with other parameters, e.g., those of the mixed product basis.
1.2.10 Denser kpoint sets
If you have time, you can try denser kpoint sets, e.g., 6×6×6.
1.2.11 Band structure calculation
The calculation of a GW band structure is not as straightforward as in the case of DFT, because we cannot arbitrarily add kpoints to the kpoint set.
On the other hand, if we restrict ourselves to the set of kpoints defined by the keyword BZ
, the band structure will have very low k resolution.
Anyway, this will be our first calculation.
1.2.11.1 Keyword PATH
Let us make a band structure calculation for the path from L over Γ to X. In principle, we would have to identify all kpoint indices along this path and define separate jobs in the job definition, but SPEX can do this for you. To define the kpoint path, add the line
KPTPATH (L,1,X)
to "spex.inp". The GW quasiparticle energies are to be calculated on each kpoint along this path.
To this end, use the keyword PATH
in the job definition, replacing the previous definition by
JOB GW PATH:(18)
(To get a somewhat finer band structure, you can also change BZ 4 4 4
to BZ 6 6 6
. If you do this change, you would have to run the oneshot FLEUR
calculation again as explained in the Sections 1.2.3 and 1.2.4, unless you use ITERATE
.)
When you now run SPEX, it will automatically choose the kpoints that lie on the path defined by KPTPATH
and calculate the respective quasiparticle energies.
Afterwards, you can extract the band structure data with the utility "spex.extr"
spex.extr g b spex.out > bandstr
(assuming that the output has been piped into the file "spex.out"), which writes the data to the file "bandstr" containing the bandstructure data as an xy dataset. The band structure can then be plotted with any plot program. In the Docker image, we have two possibilities:
 Python/Matplotlib: File → New → Console (or Notebook) → Select kernel "Python 3"
 Gnuplot: File → New → Console (or Notebook) → Select kernel "Gnuplot"
Note: We will only suggest Gnuplot plot commands in the following. Using the Gnuplot kernel, you can pretty much work as in "classic Gnuplot". For example, enter plot "bandstr" with linespoints
to plot the band structure. To submit a command, press SHIFT+ENTER. Make sure that you are in the correct directory. pwd
(same as in BASH) tells you the current directory, and cd "<path>"
(note the quotation marks!) lets you change the directory.
As expected, the band structure is not smooth. One solution would be to use denser kpoint sets, but this would entail very expensive calculations.
Note: The absolute quasiparticle energies (y axis) are plotted. The Fermi energy is not subtracted. The valenceband maximum is at around 5.2 eV.
1.2.11.2 Additional kpoints (KPT +...
)
Another possibility is to use a feature that allows us to add arbitrary kpoints, one at a time. Each additional kpoint q must be accompanied with all shifted kpoints q+k. So, for each single kpoint q we must run FLEUR to generate wavefunctions and energies at the shifted kpoint set and then run SPEX. Let us do this for a single kpoint first.
(In principle, running FLEUR can again be avoided by using ITERATE
. We leave this up to you. For simplicity, we will from now on assume that FLEUR is used for the
oneshot calculation.)
We define the conductionband minimum by
KPT +=1/2*(0,0.75,0.75)
Note: A keyword (such as KPT
) is not allowed to be used more than once in the input file. Therefore, if you already have a KPT
definition in your input file, either replace it with the one above or simply add the argument +=1/2*(0,0.75,0.75)
to it (e.g., KPT X=1/2*(0,1,1) L=1/2*(0,0,1) +=1/2*(0,0.75,0.75)
).
The label +
is a special kpoint label for additional kpoints. Then, create the kpoint file (with spex w
) and run FLEUR as explained in Sections 1.2.3 and 1.2.4.
Before running SPEX, the job definition line should be replaced by
JOB GW 1:(15) +:(15)
The bands 15 are calculated for the Γ point and the additional kpoint. After the calculation is done, you can read the fundamental band gap from the output file. It should be smaller by about 0.1 eV than the result for the Γ→X transition calculated above.
Note: It is recommendable to reduce the kpoint set to BZ 4 4 4
for the following excercises.
Let us now make a full band structure calculation for the same path as above. When you investigate your directory, you should see a file "qpts" that
was created when we last ran spex w
. This file contains a list of kpoints along the specified path. For each of the kpoints, we would have to run,
in this order, SPEX, FLEUR, and SPEX again. To simplify this task, there is a shell script called "spex.band". This shell script performs all the steps
automatically for you. The script automatically adds the RESTART
option to "spex.inp" (which tells SPEX to calculate the screened interaction only once and to
store it on disc). Specify eight bands with
JOB GW +:(18)
To run the shell script simply type spex.band
. For each kpoint, the script creates a SPEX output file "spex_NUM.out", where NUM is a counter index.
Note: If an output file (e.g., "spex_005.out") already exists in the current directory, the script "spex.band" skips the calculation of the respective kpoint and continues with the next. Therefore, do not forget to remove the files "spex_???.out" when you run "spex.band" again in a later exercise!
From the output files, the band structure data is extracted with
spex.extr g b spex_???.out > bandstr
The file "bandstr" again contains the data in the form of an xy dataset. The band structure can be plotted with one of the three possibilities described above.
Note that the conduction band minimum is, in fact, between the Γ and X point at about k=1/2*(0,0.75,0.75).
If you extract the bandstructure data, instead, with
spex.extr g b c spex_???.out > bandstr
you get an additional column, which contains the imaginary parts of the quasiparticle energies. Remember that these imaginary parts are proportional to the line width and inversely proportional to the excitation lifetime. The line widths can be plotted together with the band structure in gnuplot with
plot "bandstr" using 1:2:(6*abs($3)) with points ps var
You will see that the bands get wider the further they are away from the Fermi energy. (We will come back to this point in Section 1.2.12.)
1.2.11.3 Wannier interpolation
As a third alternative, we can use Wannier interpolation. We have to modify "spex.inp" in the following way
BZ 4 4 4
JOB GW IBZ:(14)
ITERATE
KPT X=1/2*(0,1,1) L=1/2*(0,0,1)
KPTPATH (L,1,X)
SECTION WANNIER
ORBITALS 1 4 (sp3)
MAXIMIZE
INTERPOL
END
The job definition lets SPEX calculate quasiparticle energies in the whole irreducible Brillouin zone (IBZ).
The section WANNIER
defines a set of maximally localized Wannier functions (MLWFs) of bonding sp^{3} hybrid orbitals.
These four hybrid orbitals are generated from the four lowest valence bands (first two arguments after ORBITALS
).
Running SPEX will construct the MLWFs and use them to interpolate the band structure.
The band structure data is written to the files "bands0" for the KohnSham energies and "bands1" for the GW quasiparticle energies.
Please compare the interpolated band structure stored in "bands1" and the one that we have calculated
explicitly before (file "bandstr"). (In Gnuplot: plot "bandstr","bands1" with lines
)
When you open the files with an editor, you will see that "bands1" has a column more than "bands0".
This additional column contains the interpolated imaginary parts of the quasiparticle energies.
You can plot the band structure together with the variable point size as described in the previous section.
Since we have defined four Wannier functions, we only get the four valence bands of silicon and no conduction bands.
We will need Wannier functions again when we calculate Hubbard U parameters in Section 2.
Note: If interested, you can now shortcut to other topics: (1.3) dielectric functions, (1.4) GW including spinorbit coupling, (1.5) HartreeFock and GW for metals, (2) Hubbard U parameters.
1.2.12 Spectral functions
The central quantity in the GW method is the renormalized Green function G, which is related to the selfenergy Σ through the Dyson equation G=G_{0}+G_{0}ΣG. Every GW program calculates Σ and, then, solves the Dyson equation. The last step is often simplified in such a way that an effective singleparticle equation (the socalled quasiparticle equation) is solved instead, yielding the quasiparticle energies and lifetimes instead of the Green function G, as we have done in this tutorial up to now. One can say that the quasiparticle energies and lifetimes parameterize G. This works well for the quasiparticle peaks but not for manybody states such as plasmon satellites. SPEX can also be used to calculate G directly. The imaginary part of its trace is called the spectral function. To this end, we modify "spex.inp" in the following way:
SECTION SENERGY
CONTOUR {2:2,0.01}
SPECTRAL
END
Here, we recommend to use the contour deformation method (CONTOUR
).
If you now run a GW calculation, you will get a file "spectral" containing spectral functions for each kpoint defined in the job definition. For example,
with
JOB GW +:(18)
you would get the spectral function at the additional kpoint labeled with + (presumably still the
kpoint of the conductionband minimum). Of course, you can choose any kpoint (also several kpoints).
Plot the spectral function.
The x axis is the energy axis (in eV).
One can imagine the spectral function as a vertical cut through the band structure at the respective kpoint.
If you compare with the band structure of silicon,
you will see that there is a (quasiparticle) peak in the spectral function for each band at the respective kpoint.
The peaks broaden the further they are away from the Fermi energy.
One can also observe a broad peak at around 30 eV. (You will have to reduce the y range, e.g., plot [] [:20] "spectral" with lines
to see the shallow peaks.) This is the plasmon satellite. You should see another
broad feature above the Fermi energy, at 2030 eV, which is the plasmon satellite for the inverse photoemission process.
It is also possible to calculate the spectral functions along a path in the Brillouin zone,
to obtain a renormalized band structure, in a similar way as above.
In particular, we would follow the steps of Section 1.2.11.2 or Section 1.2.11.3 with the keyword SPECTRAL
.
(With the procedure of Section 1.2.11.1 one would not get sufficient k resolution.)
Using the latter approach (Section 1.2.11.3), SPEX produces a file named "spectralw", which contains a kresolved spectral function (renormalized band structure). Do not forget to set JOB GW IBZ:(14)
and include the section WANNIER
. You can use "gnuplot" to plot the resulting renormalized band structure as a colorcoded plot by
set cbrange [0:50]
plot [] [45:5] "spectralw" palette with lines
Check if you can see the plasmon satellites in the plots! You might have to change the color range, e.g., to set cbrange [0:5]
The first approach (Section 1.2.11.2) is computationally more expensive (perhaps too expensive for the limited time of the handson session). We can use the shell script "spex.band" again, which will produce a whole series of files containing spectral functions: "spectral_001", "spectral_002", etc. Note that, if you still have files "spex_001.out" etc. in the current directory, the corresponding calculations will be skipped by the shell script. To avoid that, you should remove those files or run the script with an additional option: spex.band s
.
The corresponding data can be extracted with
spex.extr s spectral_??? > spec
and plotted in the same way as "spectralw" before.
Of course, the k resolution is not very high. One can produce a finer kmesh by KPTPATH (L,1,X) 50
(and then running
spex w
to produce the kpoint file "qpts").
The additional parameter sets the number of kpoints along a unit length. The default is 20.
1.3 Gallium nitride
GaN crystallizes in the Wurtzite (ZnS) structure. Its hexagonal unit cell ("hP" in the input for "inpgen") has the lattice parameters a=3.19 Å and c=5.189 Å with the space group 186 (P63mc). The representative atoms are at (internal coordinates):
atom species  x  y  z  Wyckoff symbol 

Ga  1/3  2/3  0.0  2b 
N  1/3  2/3  0.623  2b 
Set up the input for the material (or use the prepared one "GaN/inp_GaN"), calculate the GW band gap.
It is at the Γ point. You also need to know the band index of the valence band maximum.
Around line 80 of the SPEX output, the number of electrons in the unit cell are given (Number of valence electrons: 36
).
Dividing this number by 2 (for the two spin directions) yields the band index of the valenceband maximum.
So, the band gap is between band 18 and band 19. We are also interested in the 3d levels of Ga. These bands are the first
ten bands (each Ga atom contributes five). Just add the first band to the job definition.
GaN has twice as many atoms as silicon. So, we can reduce the BZ sampling:
BZ 3 3 3
Compare the GW band gap to the DFT one and to experiment (3.39 eV). What wavelength does a GaN laser diode have?
Compare the resulting 3d levels with the KohnSham and HF values as well as with experiment (17.1 eV). Note that the semicore level is measured with respect to the valenceband maximum (Band 18 at Γ).
It is also possible to calculate the dielectric function. For this, change the job definition to
JOB DIELEC 1:{0:30eV,0.1eV}
Here, we request a plot of the dielectric function between 0 and 5 eV with a step size of 0.01 eV. (Generally, energy values without a unit are interpreted as values in "Hartree". Alternatively, the values can be given in eV as shown.) This is done for kpoint label "1", meaning for a displacement vector of (0,0,0), corresponding to optical transitions. (The momentum of the photon is much smaller than that of the electrons. The longwavelength limit k=0 is, thus, a good approximation). A SPEX run will produce the files "dielec" and "dielecR", corresponding to the bare and the renormalized dielectric function. The optical absorption spectrum (excluding excitonic effects) can be plotted as a function of frequency ω with ("gnuplot")
plot "dielecR" using 1:3 with lines
For an electronenergy loss spectrum (EELS), we have to plot the imaginary part of the reciprocal value:
plot "dielecR" using 1:(imag(1/($2+{0,1}*$3))) with lines
The literature value of the dielectric constant (ω=0) is about 5.8. The refractive index n is the square root of the dielectric constant. The literature value is n=2.41.
1.4 Antimony telluride: Spinorbit coupling (SOC)
Let us now consider a more advanced system, in which the spinorbit coupling (SOC) plays an important role: Sb_{2}Te_{3}. The input files can be copied from here: "Sb2Te3/". These input files are especially prepared for this tutorial (to make a G^{SOC}W^{SOC} calculation possible on a slow system). You do not need to run "inpgen" this time.
Perform a GW calculation without SOC. Include the following settings in "spex.inp" for this and the next calculations:
BZ 2 2 2
NBAND 300
MEM 1500
We have not yet come across the last keyword. Setting MEM
to 1500 limits the memory usage of the SPEX run to (approximately) 1500 MB. Memory in the Docker image is rather limited. You might have to reduce the memory usage even more. Please, check your main memory in the first line of "/proc/meminfo". A reasonable value for MEM
would be about 75\% of this number.
Calculate the quasiparticle energies for the states 12 to 15 at the Γ point. The corresponding file "spex.inp" is already prepared. Calculate the value of the direct band gap, Γ→Γ, and compare the KohnSham and GW values. You can determine the band index for the valenceband maximum according to the explanation in the previous section.
Is the GW value larger or smaller than the KS one? The experimental value is under debate (170280 meV).
Now run the GW calculation with SOC. Instead of doing the oneshot calculation with FLEUR (Section 1.2.4), we use
ITERATE FR
in "spex.inp". Here, "FR" stands for "fully relativistic". So, you do not have to rerun FLEUR. This time, we have to run the executable spex.noinv
directly because the manual inclusion of SOC via ITERATE FR
is not detected by the Spex launcher script.
Remember that now the states are doubly degenerate (Kramer's degeneracy). We therefore specify the bands from 23 to 30 in the job definition. Note that the index for the valenceband maximum is doubled as well. (Spin is not a good quantum number anymore.)
Calculate again the direct GW band gap and compare with the KohnSham value and with the respective value from the previous calculation (without SOC), as well as with the experiment. Is the GW value now larger or smaller than the KohnSham one? Why?
Look at the states from 23 to 28 (the 6 highest valence bands). Has the spinorbit coupling caused a splitting of states that were otherwise degenerate?
1.5 Sodium
If you still have time, you can run a GW calculation for metallic Na. We provide "inp_Na", an input file for "inpgen", in the directory "Na/".
Try to calculate a GW band structure from P over Γ to H as explained in Section 1.2.11.2. You can use a 6×6×6 kpoint mesh and the kpoint definition
KPT P=1/4*(1,1,1) H=1/2*(1,1,1)
Sodium has four semicore states: 2s and 2p. So, the band indices in the job definition should start with 5. We intend to compare with a HartreeFock calculation. Please include the keyword LOGDIV
into the section SENERGY
(see explanation below). For extracting the band structure data use
spex.extr g b O spex_???.out > bandstr
Compare this band structure to the corresponding HartreeFock band structure (h
instead of g
after spex.extr
).
While the band gradient of the latter shows a logarithmic divergence the Fermi energy,
there is no divergent band anomaly in the GW band structure because the correlation selfenergy cancels the divergence of the HF term exactly by a term of opposite sign. The HF band anomaly in metals is unphysical. Therefore, SPEX calculates it explicitly only if the keyword LOGDIV
is specified in the input file.
Note that the band width is reduced in the GW band structure compared to the KohnSham band structure. (Can be extracted with k
after spex.extr
.)
This reduction is caused by the increased effective mass of the quasiparticles with respect to the bare electrons.
As the electron moves, it has to drag its polarization cloud (Coulomb hole) along, which makes the quasiparticle effectively heavier.
2 Hubbard U parameters
2.1 Strontium vanadate (SrVO_{3})
As a first example, we will calculate the Hubbard U (partially screened Coulomb interaction) parameter for the V t_{2g} orbitals of SrVO_{3} using the cRPA method. You find an input file ("inp_SrVO3") for "inpgen" in the directory "SrVO3/".
First, we calculate a DFT band structure with SPEX for the kpoint path RΓXMΓ.
The special kpoints are at R=(0.5,0.5,0.5), Γ=(0,0,0), X=(0.5,0,0), and M=(0.5,0.5,0).
You have to specify the keywords KPT
and KPTPATH
accordingly as described in Sec. 1.2.11. To let SPEX calculate a DFT band structure, you specify ITERATE BANDS
. SPEX will stop afterwards. The file containing the bandstructure data is called "bands". It usually contains so many bands that, in order to see separate bands, you need to decrease the plotted energy range:
plot [] [2:20] "bands" with lines
Before we calculate the Hubbard U parameter, we have to find out which correlated subspace to define. We expect it to be a vanadium d subspace, but will the subspace be composed of t_{2g} or e_{g} or all d orbitals?
To find this out, we calculate the band structure again, this time with orbital projections. This is achieved with
the keyword BANDINFO
. By default, we would get a decomposition into s, p, d, etc. orbitals of the first atom, then the second atom and so on. In the present case, we are only interested in the d states of vanadium. Therefore, we write
BANDINFO () (t2g,eg)
(the first atom is strontium, which is skipped). The next SPEX run will add orbital information to
"bands", which can be plotted with
set cbrange [0:1]
plot "bands" u 1:2:3 palette with lines
(for the t_{2g} states)
plot "bands" u 1:2:4 palette with lines
(for the e_{g} states)
You will see that the t_{2g} bands form an isolated set of bands at the Fermi energy (which is at about 12.7 eV). Therefore, we use this set of bands as the correlated subspace. The corresponding band indices are 2123. A basis set for the correlated subspace will be formed by Wannier orbitals.
Note: You can get the Fermi energy from the "out" file of FLEUR (grep fermi out
) and also from the SPEX output if you have specified a Brillouin zone sampling, e.g., BZ 4 4 4
(and without ITERATE BANDS
). Look for "Fermi energy"! The Fermi energies can be different due to the usage of different algorithms in the two codes.
Now, we calculate only the bands forming the t_{2g} subspace using Wannier interpolation, as described in 1.2.11.3. You can use the following input for "spex.inp".
BZ 4 4 4
ITERATE
KPT R=1/2*(1,1,1) G=(0,0,0) X=1/2*(1,0,0) M=1/2*(1,1,0)
KPTPATH (R,G,X,M,G)
SECTION WANNIER
ORBITALS 21 23 () (t2g)
INTERPOL
END
Note: Do not forget to run the FLEUR oneshot calculation as described in 1.2.34. Or, as a simple alternative, include the keyword ITERATE
into "spex.inp".
The Wannierinterpolated band structure is written to the file "bands0". (If you still have the keyword BANDINFO
in your input file, you will see that "bands0" also contains Wannierinterpolated orbital projections. You can plot them in the same way as "bands", and you will see that the orbital projections are practically identical to the previous result.)
You can compare the two band structures in "gnuplot":
plot "bands" with lines,"bands0" with lines
.
You might be surprised that we do not use the keyword MAXIMIZE
here. Indeed, the maximal localization procedure is often unnecessary in the case of an isolated set of bands (presently the t_{2g} bands). So, Wannier90 is not employed here.
A minimal SPEX input file for Hubbard U calculations looks like this:
BZ 3 3 3
JOB SCREENW {0}
ITERATE
SECTION SUSCEP
HUBBARD
END
SECTION WANNIER
ORBITALS 21 23 () (t2g)
END
We have reduced the Brillouinzone sampling to 3×3×3 to speed up the calculation.
The keyword SCREENW
defines the job for calculating the partially screened Coulomb potential U. The expression in the brackets {...} can be
a general frequency range (in the same form as for the dielectric function, Section 1.3). Here, {0} means to calculate the Hubbard U parameter for
the static limit (ω=0). We only need the static U for model Hamiltonians or LDA+U calculations.
In the section SUSCEP
we provide the keyword HUBBARD
to exclude transitions in the correlated subspace, otherwise SPEX calculates
the fully screened Coulomb interaction W.
Wannier functions are constructed internally in the SPEX code.
For this purpose, we have to provide the keyword ORBITALS
and specify the states from which the Wannier functions are to be constructed.
For SrVO_{3}, we construct three V t_{2g} orbitals from the states 21, 22, 23. The V atom is the second type as specified in the FLEUR input.
Therefore, we use an "empty" definition ()
for the first atom (Sr).
In this calculation, we simply use the firstguess Wannier functions, thus the keyword MAXIMIZE
is excluded.
When SPEX is run, it will calculate the Coulomb matrix U in the mixed product basis and then projects it onto the Wannier basis. You will get some effective parameters at the end of the program run, Kanamori parameters for e_{g} and t_{2g} orbitals (and HubbardHund parameters if full shells are specified). The full interaction matrix is written to the file "spex.cou".
You can run the same calculation without the keyword HUBBARD
. Do the interaction parameters differ from the previous ones? If so, why?
Let us now also include the oxygen p orbitals in the Wannier set:
SECTION WANNIER
ORBITALS 12 23 () (t2g) [p]
END
The oxygen p bands are just below the vanadium t_{2g} bands. There are nine p bands (three atoms, three p bands per atom). The square brackets [p]
define p orbitals for all three oxygen atoms. ([p]
is the same as (p) (p) (p)
.).
Calculate the Hubbard U parameters (do not forget the keyword HUBBARD
!) and compare the new U parameter for the t_{2g} states with the old one! Do they differ? What do you conclude from the difference?
Also try running a Wannier interpolation for the Wannier set including the oxygen p states and perhaps one including the e_{g} states in addition. Compare them with the standard DFT band structure ("bands").
2.2 Nickel
If you still have time, you can calculate the Hubbard U parameter for Nickel. To be more precise, we want to calculate the U parameter for the 3d states of Ni.
We can mainly proceed as in the previous section. The Brillouinzone sampling (BZ
) should be a bit finer:
BZ 6 6 6
And we need to define the Wannier orbitals (ORBITALS
) differently:
SECTION WANNIER
ORBITALS 4 9 (d)
END
Why do the band indices start with 4 in this case? (Have a look at the file "inp.xml"!)
As bulk nickel is ferromagnetic and Wannier functions have a formal spin dependence (since they are constructed from wavefunctions), we have to specify the spin indices for the Wannier products explicitly in the job definition. (Usually, the spin dependence is numerically small.)
JOB SCREENW uu{0}
Since we have defined a full shell, the 3d shell, we get HubbardHund parameters (instead of Kanamori parameters).
As the Ni d bands are entangled with the s band, the Wannier interpolation is a bit more complicated. We employ the Wannier90 library for the disentanglement procedure (FROZEN
) and for the maximal localization (MAXIMIZE
). The Wannier section could look like this:
SECTION WANNIER
ORBITALS 4 * (s,d)
MAXIMIZE
FROZEN
INTERPOL
END
Here, we have used a wildcard (*
) for the upper band index (of the Wannier window), which tells SPEX to choose a reasonable index. Also, the "frozen window" for disentanglement is chosen automatically. (It can also be set manually for fine tuning.) You can use the LΓX path KPTPATH (4,1,13)
. (Here, instead of kpoint labels, we select the kpoint indices directly. The numbers are for 6×6×6). Run a Wannier interpolation and compare with the explicit DFT band structure.