# Applications of the static sigma editor

This tutorial presents various applications of the static sigma editor. With it you can manipulate the QSGW static self-energy Σ0(k) in a variety of ways. The present tutorial displays some of the features of the editor in the context of some practical applications.

Questaal has two self-energy editors corresponding to the two forms of self energy in QSGW:

1. a static self-energy Σ0(k) that substitutes for the exchange-correlation potential in LDA, GGA, Hartree-Fock or LDA+U theory. Σ0(k) is static and hermitian, and generates a noninteracting hamiltonian H0 and a corresponding noninteracting Green’s function G0. This potential is the primary output of the the QSGW cycle and Σ0(k)  (actually $\Sigma^0{-}V_\mathrm{xc}^\mathrm{LDA}$) is saved on disk, usually in file sigm.ext. There are numerous ways to use and transform Σ0; most are accomplished through the Σ0(k) editor described here.

2. a frequency-dependent, nonhermitian self energy Σ(k,ω). The ω-dependence broadens the states; the non-hermitian part gives the quasiparticle a finite lifetime. This is not generated automatically, but must be done using a special setup, as described in this tutorial. Once the dynamical Σ(k,ω) is made, there is a corresponding dynamical self-energy editor to facilitate analysis.

Sheared GaSb

Follow the steps that create input files and carry out QSGW self-consistency in the Command Summary of the superlattice tutorial.

The do:

lmscell gasb -vz=11.5/11.43 --stack~scale=1/z~stretch=z^3~wsitex@short
lmf ctrl.gasb --wsig:fbz
cp sigm2.gasb sigm.gasb
cp sites.gasb site.gasb
lmf ctrl.gasb --rsig:fbz:rlxkp --iactiv --wsig:newkp
cp sigm2.gasb sigm.gasb
lmgwsc --wt --insul=14 --mpi=12,12 --tol=2e-5 --sym --getsigp --save= gasb-shear gasb > out.job
mpix -n 16 lmf-MPIK ctrl.gasb > out.lmf


### Preliminaries

This tutorial uses the MPI parallel form of lmf. It assumes a script mpix is present in your path that runs an MPI job with  #  processors as follows:

mpix -n # MPI-job


mpix can be simply mpirun.

If you have compiled lmf in serial mode, you can run the tutorials here in serial mode, by omitting the mpix -n #.

### Change in sigma on Shearing Zincblende GaSb

Here we demonstrate how to modify the static self-energy $\Sigma^0$ in the presence of a lattice shear. The shear reduces the symmetry, and $\Sigma^0$ must be adapted accordingly. For demonstration purposes we build on the tutorial on building superlattices, and shear GaSb in the manner used in that tutorial.

Here we modify $\Sigma^0$ calculated for the zincblende lattice, using it as a starting point for a new QSGW with the sheared lattice. Of course the entire QSGW calculation could be redone from scratch, but with small shears like this one it is a reasonable approximation that the matrix elements of $\Sigma^0$ remain invariant under the shear. Here we will check how good that ansatz is. In any case the ansatz gives a good estimate for the self-consistent $\Sigma^0$, avoiding the need to start from scratch.

To set up input conditions of the original zincblende structure, cut and paste the init file designed for InAs and GaSb shown in the superlattice tutorial and name it init.gasb. Then build the input file and carry out both LDA and QSGW self-consistency as described in detail in that tutorial. If you don’t want to read the tutorial, follow the instructions for Zincblende GaSb in the Command Summary, skipping the last part about drawing energy bands.

Assuming you followed instructions you will have run this step

mpix -n 16 lmf-MPIK ctrl.gasb > out.lmf


Find the gap :

grep gap out.lmf


You should find that the gap is 1.29 eV.

Now we are ready to make the shear. Use the superlattice editor built into lmscell to scale and stretch the lattice, so that (1) the basal plane matches that of InAs and (2) the shear conserves volume.

$lmscell gasb -vz=11.5/11.43 --stack~scale=1/z~stretch=z^3~wsitex@short  In this instruction preprocessor variable z is set to the ratio of GaSb and InAs lattice constants and: • ~scale=1/z scales the lattice constant by 1/z • ~stretch=z^3 stretches the third lattice vector by z3 • ~wsitex@short saves the modified structure into into site file sites.gasb The two scalings together preserve the volume. Compare sites.gasb to site.gasb and note that that the lattice constant has been reduced while the third lattice vector p3 has been stretched. The shear reduces the symmetry. To obtain $\Sigma^0$ for reduced symmetry, first write $\Sigma^0$ for points in the full Brillouin zone. Then for the sheared lattice, read this $\Sigma^0$ and write it again under reduced symmetry. lmf ctrl.gasb --wsig:fbz cp sigm2.gasb sigm.gasb cp sites.gasb site.gasb lmf ctrl.gasb --rsig:fbz:rlxkp --iactiv --wsig:newkp cp sigm2.gasb sigm.gasb  • Steps (1) and (2) generate a sigm.gasb without any symmetry operations. • Step (3) replaces the site containing the zincblende structure with the sheared structure • Steps (4) and (5) read sigm.gasb for the full BZ and writes it for the reduced symmetry operations of the sheared lattice. Switch --rsig:fbz:rlxkp tells lmf that $\Sigma^0(\mathbf{k})$ contains k points for the full Brillouin zone, and also to ignore the mismatch in the k sought and file values of k (k points are shifted because the lattice is sheared). Switch --wsig:newkp tells lmf to write $\Sigma^0(\mathbf{k}$ for the symmetry of the sheared mesh. Try running mpix -n 16 lmf-MPIK ctrl.gasb --rs=1,0 -vnit=1 > out &  This segment of output shows the symmetry operations of the sheared lattice is much reduced from the 24 point group operations of the Zincblende lattice:  GROUPG: the following are sufficient to generate the space group: m(1,-1,0) m(1,-1,0)  This segment of output  iors : read restart file (binary, mesh density) remesh density from 20 * 20 * 20 to 20 * 20 * 21 use from restart file: ef window, pos(sheared), pnu ignore in restart file: *  indicates the given lattice is sheared relative to the contents in the restart file, rst.gasb. lmf can nevertheless read the file. Look a little farther down can you can see the bandgap  VBmax = 0.097372 CBmin = 0.182519 gap = 0.085147 Ry = 1.15799 eV  With this ansatz for $\Sigma^0$ the gap is reduced from 1.29 eV to 1.16 eV. Now make it self-consistent lmgwsc --wt --insul=14 --mpi=12,12 --tol=2e-5 --sym --getsigp --save= gasb-shear gasb > out.job mpix -n 16 lmf-MPIK ctrl.gasb > out.lmf  You should find that the ansatz for $\Sigma^0$ is very close to the self-consistent one: $ grep more out.job
lmgwsc : iter 1 of 999  RMS change in sigma = 2.53E-05  Tolerance = 2e-5  more=T ...
lmgwsc : iter 2 of 999  RMS change in sigma = 5.58E-06  Tolerance = 2e-5  more=F ...


Run lmf with the self-consistent $\Sigma^0$ and find the gap once again

mpix -n 16 lmf-MPIK ctrl.gasb --rs=1,0 -vnit=1 > out.lmf &
grep gap out.lmf


### Reading and writing Real-Space sigm Files

Files associated with this tutorial are too large to be included with the basic distribution. Download and unpack the tarball FeTutorial.tar.gz and change to directory fetutorial.

  cp input/* .
cp bas2.tppc3.tpd4.sep12/rst.fe .
cp bas2.tppc3.tpd4.sep12/nk12/* .
ln -s -f sigm sigm.fe


If your computer hardware does not not-use-ieee-conventions-for-binary-files, see here before proceeding.

Verify that the input conditions result in a self-consistent potential.

  rm -f mixm.fe log.fe
mpix -n 12 lmf -vnit=1 --rs=1,0 ctrl.fe cat switches-for-lm > out.noso.001


If you want to carry out a corresponding LDA calculation, do:

  cp bas2.tppc3.tpd4.sep12/rst.lda rst.fe
rm -f mixm.fe log.fe
mpix -n 12 lmf -vsig=0 -vnit=1 --rs=1,0 ctrl.fe cat switches-for-lm > out.noso.lda.001


Problems arise if you try to include use the given self-energy in a calculation with lower symmetry, e.g. a shear distortion or the addition of spin-orbit coupling. It immediately appears if you repeat the calculation lmf with spin-orbit coupling on (note tag  SO  in the input file!):

  lmf -vso=1 -vnit=1 --rs=1,0 ctrl.fe cat switches-for-lm


This is because by the given Σ0xc is stored on a mesh of k-points. SO coupling reduces symmetry and requires that Σ0 be available on a different k-mesh.

You can work around this by saving Σ0 in a real space form; that is, map Σ0RL,RL(k) to Σ0RL,RL(T) = Σ0R+TL,RL. T is a crystal lattice translation vector; there are as many T-points as k points in k-space. The transformation is exact and no information is lost. (It is accomplished in practice using FFT techniques, and the original Σ0RL,RL(k) can be recovered by a Bloch sum.) Once generated Σ0(T) doesn’t depend on the number of k-points, so it is preferable in this context even while it is less compact.

### Computers that do not use IEEE conventions for binary files.

Files sigm._ext and rst._ext are stored in binary form following IEEE conventions. If your computer doesn’t read binary files in this format, do the following additional steps:

  cp bas2.tppc3.tpd4.sep12/rsta.fe.gz .
gunzip rsta.fe.gz
gunzip sigma.fe.gz
lmf -vnit=0 --rs=2,1 ctrl.fe cat switches-for-lm
lmf ctrl.fe cat switches-for-lm --rsig:ascii --wsig
cp sigm2.fe sigm


For the GaSb shear demo, compare energy bnds for the ansatz $\Sigma^0$ and the self-consistent one. You should find them nearly identical.