Quasiparticle spectrum in the GW approximation: tutorial¶
For a brief introduction to the GW theory and the details of its implementation in GPAW, see Quasiparticle spectrum in the GW approximation: theory.
More information can be found here:
F. Hüser, T. Olsen, and K. S. Thygesen
Quasiparticle GW calculations for solids, molecules, and twodimensional materials
Physical Review B, Vol. 87, 235132 (2013)
Quasiparticle spectrum of bulk diamond¶
In the first part of the tutorial, the G0W0 calculator is introduced and the quasiparticle spectrum of bulk diamond is calculated.
Groundstate calculation¶
First, we need to do a regular groundstate calculation. We do this in plane wave mode and choose the LDA exchangecorrelation functional. In order to keep the computational efforts small, we start with (3x3x3) kpoints and a plane wave basis up to 300 eV.
from ase.build import bulk
from gpaw import GPAW, FermiDirac
from gpaw.wavefunctions.pw import PW
a = 3.567
atoms = bulk('C', 'diamond', a=a)
calc = GPAW(
mode=PW(300), # energy cutoff for plane wave basis (in eV)
kpts={'size': (3, 3, 3), 'gamma': True},
xc='LDA',
occupations=FermiDirac(0.001),
txt='C_groundstate.txt'
)
atoms.set_calculator(calc)
atoms.get_potential_energy()
calc.diagonalize_full_hamiltonian() # determine all bands
calc.write('C_groundstate.gpw','all') # write out wavefunctions
It takes a few seconds on a single CPU. The last line in the script creates a .gpw file which contains all the informations of the system, including the wavefunctions.
Note
You can change the number of bands to be written out by using
calc.diagonalize_full_hamiltonian(nbands=...)
.
This can be useful if not all bands are needed.
The GW calculator¶
Next, we set up the G0W0 calculator and calculate the quasiparticle spectrum
for all the kpoints present in the irreducible Brillouin zone from the ground
state calculation and the specified bands.
In this case, each carbon atom has 4 valence electrons and the bands are double
occupied. Setting bands=(3,5)
means including band index 3 and 4 which is
the highest occupied band and the lowest unoccupied band.
from gpaw.response.g0w0 import G0W0
gw = G0W0(calc='C_groundstate.gpw',
nbands=30, # number of bands for calculation of selfenergy
bands=(3, 5), # VB and CB
ecut=20.0, # planewave cutoff for selfenergy
filename='Cg0w0',
savepckl=True) # save a .pckl file with results
result = gw.calculate()
It takes about 30 seconds on a single CPU for the
calculate()
method to finish:

G0W0.
calculate
()[source]¶ Starts the G0W0 calculation.
Returns a dict with the results with the following key/value pairs:
key value f
Occupation numbers eps
KohnSham eigenvalues in eV vxc
Exchangecorrelation contributions in eV exx
Exact exchange contributions in eV sigma
Selfenergy contributions in eV dsigma
Selfenergy derivatives sigma_e
Selfenergy contributions in eV used for ecut extrapolation Z
Renormalization factors qp
Quasi particle (QP) energies in eV iqp
GW0/GW: QP energies for each iteration in eV All the values are
ndarray
’s of shape (spins, IBZ kpoints, bands).
The dictionary is stored in Cg0w0_results.pckl
. From the dict it is
for example possible to extract the direct bandgap at the Gamma point:
import pickle
import numpy as np
results = pickle.load(open('Cg0w0_results.pckl', 'rb'))
direct_gap = results['qp'][0, 0, 1]  results['qp'][0, 0, 2]
print('Direct bandgap of C:', direct_gap)
with the result: 6.96 eV.
The possible input parameters of the G0W0 calculator are listed here:

class
gpaw.response.g0w0.
G0W0
(calc, filename='gw', restartfile=None, kpts=None, bands=None, relbands=None, nbands=None, ppa=False, xc='RPA', fxc_mode='GW', density_cut=1e06, do_GW_too=False, av_scheme=None, Eg=None, truncation=None, integrate_gamma=0, ecut=150.0, eta=0.1, E0=27.211386024367243, domega0=0.025, omega2=10.0, q0_correction=False, anisotropy_correction=None, nblocks=1, savew=False, savepckl=True, maxiter=1, method='G0W0', mixing=0.2, world=<gpaw.mpi.SerialCommunicator object>, ecut_extrapolation=False, nblocksmax=False, gate_voltage=None)[source]¶ G0W0 calculator.
The G0W0 calculator is used is used to calculate the quasi particle energies through the G0W0 approximation for a number of states.
 calc: str or PAW object
 GPAW calculator object or filename of saved calculator object.
 filename: str
 Base filename of output files.
 restartfile: str
 File that stores data necessary to restart a calculation.
 kpts: list
 List of indices of the IBZ kpoints to calculate the quasi particle energies for.
 bands: tuple of two ints
 Range of band indices, like (n1, n2), to calculate the quasi particle energies for. Bands n where n1<=n<n2 will be calculated. Note that the second band index is not included.
 relbands: tuple of two ints
 Same as bands except that the numbers are relative to the number of occupied bands. E.g. (1, 1) will use HOMO+LUMO.
 ecut: float
 Plane wave cutoff energy in eV.
 ecut_extrapolation: bool or array
 If set to True an automatic extrapolation of the selfenergy to infinite cutoff will be performed based on three points for the cutoff energy. If an array is given, the extrapolation will be performed based on the cutoff energies given in the array.
 nbands: int
 Number of bands to use in the calculation. If None, the number will be determined from :ecut: to yield a number close to the number of plane waves used.
 ppa: bool
 Sets whether the GodbyNeeds plasmonpole approximation for the dielectric function should be used.
 xc: str
 Kernel to use when including vertex corrections.
 fxc_mode: str
 Where to include the vertex corrections; polarizability and/or selfenergy. ‘GWP’: Polarizability only, ‘GWS’: Selfenergy only, ‘GWG’: Both.
 density_cut: float
 Cutoff for density when constructing kernel.
 do_GW_too: bool
 When carrying out a calculation including vertex corrections, it is possible to get the standard GW results at the same time (almost for free).
 av_scheme: str
 ‘wavevector’. Method to construct kernel. Only ‘wavevector’ has been tested and works here. The implementation could be extended to include the ‘density’ method which has been tested for total energy calculations (rALDA etc.)
 Eg: float
 Gap to apply in the ‘JGMs’ (simplified jelliumwithgap) kernel. If None the DFT gap is used.
 truncation: str
 Coulomb truncation scheme. Can be either wignerseitz, 2D, 1D, or 0D
 integrate_gamma: int
 Method to integrate the Coulomb interaction. 1 is a numerical integration at all qpoints with G=[0,0,0]  this breaks the symmetry slightly. 0 is analytical integration at q=[0,0,0] only  this conserves the symmetry. integrate_gamma=2 is the same as 1, but the average is only carried out in the nonperiodic directions.
 E0: float
 Energy (in eV) used for fitting in the plasmonpole approximation.
 domega0: float
 Minimum frequency step (in eV) used in the generation of the non linear frequency grid.
 omega2: float
 Control parameter for the nonlinear frequency grid, equal to the frequency where the grid spacing has doubled in size.
 gate_voltage: float
 Shift Fermi level of ground state calculation by the specified amount.
 q0_correction: bool
 Analytic correction to the q=0 contribution applicable to 2D systems.
 anisotropy_correction: bool
 Old term for the q0_correction.
 nblocks: int
 Number of blocks chi0 should be distributed in so each core does not have to store the entire matrix. This is to reduce memory requirement. nblocks must be less than or equal to the number of processors.
 nblocksmax: bool
 Cuts chi0 into as many blocks as possible to reduce memory requirements as much as possible.
 savew: bool
 Save W to a file.
 savepckl: bool
 Save output to a pckl file.
 method: str
 G0W0 or GW0(eigenvalue selfconsistency in G) currently available.
 maxiter: int
 Number of iterations in a GW0 calculation.
 mixing: float
 Number between 0 and 1 determining how much of previous iteration’s eigenvalues to mix with.
 ecut_extrapolation: bool
 Carries out the extrapolation to infinite cutoff automatically.
Convergence with respect to cutoff energy and number of kpoints¶
Can we trust the calculated value of the direct bandgap? Not yet. A check for
convergence with respect to the plane wave cutoff energy and number of k
points is necessary. This is done by changing the respective values in the
groundstate calculation and restarting. Script
C_ecut_k_conv_GW.py
carries out the calculations and
C_ecut_k_conv_plot_GW.py
plots the resulting data. It takes
several hours on a single xeon8 CPU (8 cores). The resulting figure is
shown below.
A kpoint sampling of (8x8x8) seems to give results converged to within 0.05 eV.
The plane wave cutoff is usually converged by employing a \(1/E^{3/2}_{\text{cut}}\) extrapolation.
This can be done with the following script: C_ecut_extrap.py
resulting
in a direct band gap of 7.57 eV. The extrapolation is shown in the figure below
Frequency dependence¶
Next, we should check the quality of the frequency grid used in the
calculation. Two parameters determine how the frequency grid looks.
domega0
and omega2
. Read more about these parameters in the tutorial
for the dielectric function Frequency grid.
Running script C_frequency_conv.py
calculates the direct band
gap using different frequency grids with domega0
varying from 0.005 to
0.05 and omega2
from 1 to 25. The resulting data is plotted in
C_frequency_conv_plot.py
and the figure is shown below.
Converged results are obtained for domega0=0.02
and omega2=15
, which
is close to the default values.
Final results¶
A full G0W0 calculation with (8x8x8) kpoints and extrapolated to infinite cutoff results in a direct band gap of 7.57 eV. Hence the value of 6.96 eV calculated at first was not converged!
Another method for carrying out the frequency integration is the Plasmon Pole
approximation (PPA). Read more about it here Plasmon Pole Approximation. This is
turned on by setting ppa = True
in the G0W0 calculator (see
C_converged_ppa.py
). Carrying out a full \(G_0W_0\) calculation with the PPA
using (8x8x8) kpoints and extrapolating from calculations at a cutoff of 300
and 400 eV gives a direct band gap of 7.52 eV, which is in very good agreement
with the result for the full frequency integration but the calculation took
only minutes.
Note
If a calculation is very memory heavy, it is possible to set nblocks
to an integer larger than 1 but less than or equal to the amount of CPU
cores running the calculation. With this, the response function is divided
into blocks and each core gets to store a smaller matrix.
Quasiparticle spectrum of twodimensional materials¶
Carrying out a G0W0 calculation of a 2D system follows very much the same recipe
as outlined above for diamond. To avoid having to use a large amount of vacuum in
the outofplane direction we advice to use a 2D truncated Coulomb interaction,
which is turned on by setting truncation = '2D'
. Additionally it is possible
to add an analytical correction to the q=0 term of the Brillouin zone sampling
by specifying q0_correction=True
. This means that a less dense kpoint
grid will be necessary to achieve convergence. More information about this
specific method can be found here:
F. A. Rasmussen, P. S. Schmidt, K. T. Winther and K. S. Thygesen
Physical Review B, Vol. 94, 155406 (2016)
How to set up a 2D slab of MoS2 and calculate the band structure can be found in
MoS2_gs_GW.py
. The results are not converged but a band gap of 2.57 eV is obtained.
Including vertex corrections¶
Vertex corrections can be included through the use of a xc kernel known from TDDFT. The vertex corrections can be included in the polarizability and/or the selfenergy. It is only physically well justified to include it in both quantities simultaneously. This leads to the \(GW\Gamma\) method. In the \(GW\Gamma\) method, the xc kernel mainly improves the description of shortrange correlation which manifests itself in improved absolute band positions. Only including the vertex in the polarizability or the selfenergy results in the \(GWP\) and \(GW\Sigma\) method respectively. All three options are available in GPAW. The shorthand notation for the selfenergy in the four approximations available is summarized below:
More information can be found here:
P. S. Schmidt, C. E. Patrick, and K. S. Thygesen
Simple vertex correction improves GW band energies of bulk and twodimensional crystals
To appear in Physical Review B.
Note
Including vertex corrections is currently not possible for spinpolarized systems.
A \(GW\Gamma\) calculation requires that 3 additional keywords are specified in the GW calculator:
 Which kernel to use:
xc='rALDA'
,xc='rAPBE'
etc..  How to construct the kernel:
av_scheme='wavevector'
orav_scheme='density'
. The wavevector scheme is preferred here.  How to apply the kernel:
fxc_mode = 'GWG'
,fxc_mode='GWP'
orfxc_mode='GWS'
.
Carrying on from the ground state calculation in MoS2_gs_GW.py
, a \(GW\Gamma\) calculation can be done with the following script: MoS2_GWG.py
.
The \(GW\) and \(GW\Gamma\) band structures can be visualized with the MoS2_bs_plot.py
script resulting in the figure below. Here, the effect of the vertex is to shift the bands upwards by around 0.5 eV whilst leaving the band gap almost unaffected.
Note
When carrying out a \(G_0W_0\Gamma\) calculation by specifying the 3 keywords above, the do_GW_too = True
option allows for a simultaneous \(G_0W_0\) calculation. This is faster than doing two seperate calculations as \(\chi_0\) only needs to be calculated once, but the memory requirement is twice that of a single \(G_0W_0\) calculation. The \(G_0W_0\Gamma\) results will by default be stored in g0w0_results.pckl and the \(G_0W_0\) results in g0w0_results_GW.pckl. The results of both calculations will be printed in the output .txt file.
GW0 calculations¶
It is currently possible to add eigenvalue selfconsistency in the Green’s function.
This is activated by setting method='GW0'
, specifying how many iterations you
are interested in, maxiter=5
and optionally also how much of the previous iteration’s
eigenvalues you want mixed in, mixing=0.5
. Usually 5 iterations are enough to reach
convergence with a mixing of 50%. Only the bands specified will be updated selfconsistently.
The bands above(below) the highest(lowest) included band will be shifted with the
kpoint averaged shift of the band below(above).
The results after each iteration is printed in the output file.
The following script calculates the band gap of bulk BN within the GW0 approximation:
BN_GW0.py
. The figure below shows the value of the gap during the first five
iterations, where iteration zero is the DFT gap (BN_GW0_plot.py
).
Note: The calculations are not converged with respect to kpoints, frequency points or cutoff energy.