Tutorial

In the tutorial folder you can find some examples of how to get started using yambopy. The first step in any calculation with yambo is to calculate the ground state proprieties using either abinit or pw.x. We don’t have support to read and write abinit input files. To do that you should use the abipy package. Included in the yambopy package we include some basic scripts to generate Quantum Espresso input files.

GW convergence (Si)

by H. Miranda

1. Ground State

Go to the tutorial folder and run the ground state calculation using the gs_si.py file:

python gs_si.py
python gs_si.py -r -s -n

When you run the script without any arguments it will display the available options. The second line will run a relaxation of the structure (-r, relax option), read the optimized cell parameter and create a new input file that is used to run a self-consistent (-s, –scf option) cycle and a non self-consistent (-n, –nscf option) cycle using the charge density calculated on the previous run.

2. GW convergence

Afterwards you can run a GW calculation using the gw_si.py script and a Bethe-Salpether (BSE) calculation using the bse_si.py. In the beginning of each script (for GW or BSE) there is a check for the presence of the SAVE database. In case it is not present it will be generated.

In the gw_conv_si.py you will find an example of how to use the optimize() function to converge the calculation parameters.

#create the yambo input file
y = YamboIn('%s -d -g n -V all'%yambo,folder='gw_conv')
y['QPkrange'][0][2:4] = [6,10]
conv = { 'FFTGvecs': [[10,15,20],'Ry'],
         'NGsBlkXd': [[1,2,5], 'Ry'],
         'BndsRnXd': [[1,10],[1,20],[1,30]] }

def run(filename):
    """ Function to be called by the optimize function """
    folder = filename.split('.')[0]
    print(filename,folder)
    os.system('cd gw_conv; yambo -F %s -J %s -C %s 2> %s.log'%(filename,folder,folder,folder))

y.optimize(conv,run=run)

This code will run yambo as many times as variables specified in the conv dictionary. The first calculation is called reference and uses as parameters the first element of each of the lists. For each of the other elements of the list a calculation is made.

3. Collect the data

Once all the calculations are finished it’s time to pack all the files in the json format for posterior analysis. For this use the YamboOut() class:

#pack the files in .json files
for dirpath,dirnames,filenames in os.walk('gw_conv'):
  #check if there are some output files in the folder
  if ([ f for f in filenames if 'o-' in f ]):
      y = YamboOut(dirpath,save_folder=dirpath)
      y.pack()

This snippet of code can be called using the function:

pack_files_in_folder('gw_conv',save_folder='gw_conv')

4. Plot the data

After this you should have a set of json files in the folder, one for each calculation. To make a plot of them all you just need to run:

#plot the results using yambo analyser
y = YamboAnalyser('gw_conv')
y.plot_gw('qp')
path = [[[0.5,   0,   0],'L'],
        [[  0,   0,   0],'$\Gamma$'],
        [[  0, 0.5, 0.5],'X'],
        [[1.0, 1.0, 1.0],'$\Gamma$']]
ya.plot_gw_path('qp',path)

You can add more plots by simply adding more files in the folder you give as input to the YamboAnalyser() class. At the end you should obtain a plot like this:

_images/gw_si.png

Coulomb-cutoff (BN)

by H. Miranda

In this example we will test the convergence of the coulomb truncation for a BSE calculation in single layer Boron Nitride. For that we define a loop where we perform a self-consistent ground state calculation, non self-consistent calculation, create the databases and run yambo with increasing vacuum and plot the absorption spectra.

2. Coulomb truncation convergence

In the folder tutorials/bn/ you find the python script bse_cutoff.py. You can run this script with:

python bse_cutoff.py -r

The main loop changes the layer_separation variable using values from a list. In the script you can find how the functions scf, ncf and database are defined.

#for each separation run the ground state calculation and
for layer_separation in layer_separations:

  root_folder = "%s/%d"%(work_folder,layer_separation)
  if not os.path.isdir(root_folder):
      os.makedirs(root_folder)

  # run the ground state calculation
  print("scf cycle")
  scf(layer_separation,folder="%s/scf"%root_folder)
  os.system("cd %s/scf; pw.x < %s.scf > scf.log"%(root_folder,prefix))

  # run the non self consistent calculation
  print("nscf cycle")
  src ='%s/scf/%s.save'%(root_folder,prefix)
  dst ='%s/nscf/%s.save'%(root_folder,prefix)
  nscf(layer_separation,folder="%s/nscf"%root_folder)
  os.system( 'cp -r %s %s'%(src,dst) )
  os.system("cd %s/nscf; pw.x < %s.nscf > nscf.log"%(root_folder,prefix))

  # generate the database
  database('%s'%root_folder,nscf_folder="%s/nscf"%root_folder)

  # calculate the absorption spectra using yambo
  y = YamboIn('yambo -r -b -o b -k sex -y d -V all',folder=root_folder)

  y['FFTGvecs'] = [30,'Ry']
  y['NGsBlkXs'] = [1,'Ry']
  y['BndsRnXs'] = [1,30]

  y['CUTGeo'] = 'box z'
  y['CUTBox'] = [0,0,layer_separation-1]

  y['KfnQP_E']  = [1.0,1.0,1.0] #scissor operator
  y['BSEBands'] = [3,6]
  y['BEnSteps'] = 500
  y['BEnRange'] = [[1.0,6.0],'eV']
  y.write('%s/yambo_run.in'%root_folder)
  os.system('cd %s; %s -F yambo_run.in -J %d'%(root_folder,yambo,layer_separation))

3. Plot the convergence

You can plot the results using:

python bse_cutoff.py -p

You should obtain a plot like this:

_images/bse_cutoff.png

Parallel Bethe-Salpeter (MoS2)

by H. Miranda

Warning

The merge_eps.py gathers the dielectric function databases into one folder with the correct numeration. This script should be used in versions of yambo that create one file ndb.em1*_fragment_1 per folder. In the next version of yambo the files will already have the correct numeration so you just need to collect all the files.

In this tutorial we will show how you can parallelize the dielectric function calculation in separate jobs for a BSE optical absorption spectra calculation.

The idea is that in certain clusters its advantageous to split the dielectric function calculation in smaller jobs (one for each q-point) that can run at the same time. Using the yambo parallelization you can separate the dielectric function calculation among many cpus using the variable q in X_all_q_CPU and X_all_q_ROLEs. The issue is that you still need to make a big reservation and in some cases there is load imbalance (some nodes end up waiting for others). Splitting in smaller jobs can help your jobs to get ahead in the queue and avoid the load imbalance. If there are many free nodes you might end up running all the q-points at the same time.

The idea is quite simple: you create an individual input file for each q-point, submit each job separately, collect the results and do the final BSE step (this method should also apply for a GW calculation).

1. Ground State

The ground state calculation for MoS:sub:2 is made in a similar fashion as the previous examples. If some of the steps are already calculated you can tell the script not to run them using for example:

python gs_mos2.py -n2

The option -n2 will tell the script not to run the double grid nscf calculation.

2. Parallel Dielectric function

Here we tell yambo to calculate the dielectric function. We read the number of q-points the system has and generate one input file per q-point. Next we tell yambo to calculate the first q-point. Yambo will calculate the dipoles and the dielectric function at the first q-point. Once the calculation is done we copy the dipoles to the SAVE directory. After that we can run each q-point calculation as a separate job. Here the user can decide to submit one job per q-point on a cluster or use a program like gnuparallel to schedule the jobs according to the available resources. In this example we use the second option.

#create the yambo input file
y = YamboIn('yambo -r -b -o b -V all',folder='bse_par')
y['FFTGvecs'] = [15,'Ry']
y['NGsBlkXs'] = [1,'Ry']
y['BndsRnXs'] = [[1,40],'']
y.write('bse_par/yambo_run.in')

#get the number of q-points
_,nkpoints = y['QpntsRXs'][0]

#prepare the q-points input files
f = open('jobs.sh','w')
for nk in xrange(1,int(nkpoints)+1):
    y['QpntsRXs'] = [[nk,nk],'']
    y.write('bse_par/yambo_q%d.in'%(nk))
    if nk != 1:
        f.write('cd bse_par; %s -F yambo_q%d.in -J %d\n'%(yambo,nk,nk))
f.close()

#calculate first q-point and dipoles
os.system('cd bse_par; %s -F yambo_q1.in -J 1'%yambo)
#copy dipoles to save
os.system('cp bse_par/1/ndb.dip* bse_par/SAVE')
#run jobs using gnuparallel
os.system('parallel :::: jobs.sh')

3. BSE

Once the dielectric function is calculated its time to collect the data in one folder and do the last step of the calculation: generate the BSE Hamiltonian, diagonalize it and calculate the absorption.

#gather all the files
os.system('cp merge_eps.py bse_par')
os.system('cd bse_par; python merge_eps.py')

y = YamboIn('yambo -r -b -o b -k sex -y d -V all',folder='bse_par')
y['FFTGvecs'] = [15,'Ry']
y['NGsBlkXs'] = [1,'Ry']
y['BndsRnXs'] = [[1,40],'']
y['BSEBands'] = [8,11]
y['BEnSteps'] = 500
y['BEnRange'] = [[1.0,6.0],'eV']
y.arguments.append('WRbsWF')

y.write('bse_par/yambo_run.in')
os.system('cd bse_par; %s -F yambo_run.in -J yambo'%yambo)

3. Collect and plot the results

You can plot the data much in the same way as you did for the GW calculation.

#collect the data
pack_files_in_folder('bse_par')

#plot the results using yambo analyser
y = YamboAnalyser('bse_par')
y.plot_bse('eps')

You should obtain a plot like this:

_images/bse_mos2.png

Excitonic wavefunctions (BN)

by H. Miranda

In this example we show how to use the yambopy to plot the excitonic wavefunctions that result from a BSE calculation. Beaware the parameters of the calculation are not high enough to obtain a converged calculation. To run the calculation do:

python gs_bn.py -s -n
python bse_bn.py -r

Afterwards you can run a basic analysis of the excitonic states and store the wavefunctions of the ones that are more optically active and plot their wavefunctions in reciprocal space. Plots in real space are also possible using yambopy but won’t be treated here. In the analysis code you have:

#get the absorption spectra
a = YamboBSEAbsorptionSpectra('yambo',save='bse/SAVE',path='bse')
excitons = a.get_excitons(min_intensity=0.0005,max_energy=6,Degen_Step=0.01)
print( "nexcitons: %d"%len(excitons) )
print( "excitons:" )
print( excitons )
a.get_wavefunctions(Degen_Step=0.01,repx=range(-1,2),repy=range(-1,2),repz=range(1))
a.write_json()

The class YamboBSEAbsorptionSpectra() reads the absoprtion spectra obtained with explicit diagonalization of the BSE matrix. yambo if the job_string identifier used when running yambo, bse is the name of the folder where the job was run. The function get_excitons() runs ypp to obtain the exitonic states and their intensities. The function get_wavefunctions() also calls ypp and reads the reciprocal (and optionally real space) space wavefunctions and finally we store all the data in a json file.

This file can then be easily ploted with another python script. To run this part of the code you can do:

python bse_bn.py -a
python plot_excitons.py

You should then obtain plots similiar (these ones were generated on a 30x30 kpoint grid) to the figures presented here:

_images/absorption_bn.png _images/excitons_bn.png

Again beaware this figures serve only to show the kind of representation that can be obtained with yambo and yambopy. Further convergence tests need to be performed to obtain accurate results, but that is left to the user.

Some plots of excitonic wavefunctions in real space are show in a parallel project in: http://henriquemiranda.github.io/excitonwebsite/

Real Time Simulations (Si)

by A. Molina Sánchez

We start with the calculation of the ground state properties using the script gs_si.py in the tutorials/si folder. We will create self-consistent data (folder scf) and a non-self consistent data (folder nscf). All the real-time calculations are realized inside the folder rt.

In order to perform real-time simulations we need to perform some preliminary steps:

  • Creating the files containing the electron-phonon matrix elements: We use

quantum espresso (‘ph.x’). The grid used for obtaining the eletron-phonon matrix elements must be the same than for the real-time simulations. See in the yambo website more information about the methodology.

python gkkp_si.py

The script will create a folder GKKP inside rt. GKKP contains all the electron-phonon matrix elements in the full Brillouin zone.

  • Breaking symmetries. The action of an external field breaks the symmetry of

the system. We need to break the symmetries according with the direction of the polarization of the incident light. When we run for first time:

python rt_si.py

yambopy check if the SAVE exists inside rt. If not, it breaks the symmetries. We can select linear or circular polarized light. The light polarization must be the same along all the calculations. Here we select a field along x-axis:

ypp['Efield1'] = [ 1, 0, 0]  # Field in the X-direction

The circular polarized field must be set as follows:

ypp['Efield1'] = [ 1, 0, 0]  # Circular polarization
ypp['Efield2'] = [ 0, 1, 0]

If everything is OK we have to find inside rt the folder SAVE and GKKP. Now we can start the real-time simulations. We discuss the following run levels.

1. Collisions.

yambo -r -e -v c -V all

Calculation of the collisions files. This step is mandatory to run any real-time simulation. We calculate the matrix elements related with the electronic correlation (see Ref. PRB 84, 245110 (2011)). We have several choices for the potential approximation (we use COHSEX in this tutorial).

run['HXC_Potential'] = 'COHSEX' # IP, HARTREE, HARTREE-FOCK, COHSEX

The variables for the collisions are very similar to a Bethe-Salpeter (BSE) run. First, we start calculating the static dielectric function. It follows the calculation of the Kernel components for the electron-hole states of interest. In addition, we have several cutoffs to be set, in a similar way than in the case of the BSE.

run['NGsBlkXs']  = [100,'mHa']  # Cut-off of the dielectric function
run['BndsRnXs' ] = [1,30]       # Bands of the dielectric function
run['COLLBands'] = [2,7]        # States participating in the dynamics.
run['HARRLvcs']  = [5,'Ha']     # Hartree term: Equivalent to BSENGexx in the BSE run-level
run['EXXRLvcs']  = [100,'mHa']  # Forck term:   Equivalent to BSENGBlk in the BSE run-level
run['CORRLvcs']  = [100,'mHa']  # Correlation term: Not appearing in BSE.

In general, we use the converged parameters of the BSE to set the variables of the collisions run. For parallel runs (see section for parallel advices) a common recipe is to parallelize only in k points.

2. Time-dependent with a delta pulse.

yambo -q p

The delta pulse real time simulation is the equivalent to the Bethe-Salpeter equation in the time domain (if we use the COHSEX potential). We have to set the propagation variables: (i) time interval, (ii) duration of the simulation, and (iii) integrator. We have also to set the intensity of the delta pulse.

run['GfnQP_Wv']   = [0.10,0.00,0.00]    # Constant damping valence
run['GfnQP_Wc']   = [0.10,0.00,0.00]    # Constant damping conduction

run['RTstep']      = [ 100 ,'as']  # Interval
run['NETime']      = [ 300 ,'fs']  # Duration
run['Integrator']  = "RK2 RWA"     # Runge-Kutta propagation

run['Field1_kind'] = "DELTA"          # Type of pulse
run['Field1_Int']  = [ 100, 'kWLm2']  # Intensity pulse

run['IOtime']      = [ [0.050, 0.050, 0.100], 'fs' ]

The IOtime intervals specify the time interval to write (i) carriers, (ii) green’s functions and (iii) output. In general, we can set high values to avoid frequent IO and hence slow simulations. Only in the case where we need the data to calculate the Fourier Transform (as in the case of the delta pulse, we set this variable to lower values). The constant dampings GfnQP_Wv and GfnQP_Wc are dephasing constants, responsible of the decaying of the polarization. They are the finite-time equivalent to the finite broadening of the Bethe-Salpeter solver (BDmRange).

A mandatory test to check if yambo_rt is running properly is to confront the BSE spectra with the obtained using yambo_rt (use the script kbe-spectra.py). Observe how the KBE spectra is identical to the BSE spectra except for intensities bigger than 1E5. Beyond this value we are not longer in the linear response regime.

_images/bse-kbe-intensity.png

3. Time-dependent with a gaussian pulse.

yambo -q p

The run-level is identical for that of the delta pulse. However, we have to set more variables related with the pulse kind. In order to generate a sizable amount of carriers, the pulse should be centered at the excitonic peaks (obtained from the delta pulse spectra). The damping parameter determines the duration of the pulse. We can also chose linear or circular polarization (see later the section for circular polarization). Be aware of setting the duration of the simulation accordingly with the duration of the pulse.

run['Field1_kind'] = "QSSIN"
run['Field1_Damp'] = [  50,'fs']         # Duration of the pulse
run['Field1_Freq'] = [[2.3,2.3],'eV']    # Excitation frequency
run['Field1_Int']  = [ 1, 'kWLm2']       # Intensity pulse

In general, for any pulse create a population of carriers (electron-holes). One sign that simulation is running well is that the number of electrons and holes is the same during all the simulation. Below we show the typical output for a simulation of a gaussian pulse, the number of carriers increases until the intensity of the pulse becomes zero.

_images/qssin-pulse.png

Besides the delta and gaussian pulse we can use others as the sin pulse. Below we have a brief summary of the three pulses, showing the external field and the number of carriers. Observe than the sinusoidal pulse is active along all the simulation time, therefore we are always creating carriers. After certain time the number of electrons will exceed the charge acceptable in a simulation of linear response. The polarization follows the field. In the case of the delta pulse, we see a zero-intensity field and a constant number of carriers. Thus, the pulse is only active at the initial time and afterwards the polarization decays due to the the finite lifetime given by GfnQP_Wv and GfnQP_Wc.

_images/dyn-field-pulses.png

4. Time-dependent with a gaussian pulse and dissipation

The Kadanoff-Baym equation implemented in yambo includes dissipation mechanisms such as (i) electron-phonon scattering, (ii) electron-electron scattering and (iii) electron-photon scattering. In the following subsections we use a gaussian pulse with the parameters given above.

4.1 Electron-phonon interaction

yambo -q p -s p

In order to include electron-phonon dissipation, previously we need to create the electron-phonon matrix elements. We call the script gkkp_sii.py. We can check

python gkkp_si.py

This script runs QE to calculate the matrix elements and then ypp_ph to convert them to the yambo format. If everything is right we find a folder call GKKP inside rt. GKKP contains all the electron-phonon matrix elements in the full Brillouin zone. The variables related to the dissipation are

run['LifeExtrapSteps'] = [ [1.0,1.0], 'fs' ]
run['BoseTemp']        = [ 0, 'K']
run['ElPhModes']       = [ 1, 9]
run.arguments.append('LifeExtrapolation')     # If commented:   Lifetimes are constant

The variable LifeExtrapSteps sets the extrapolation steps to calculate the electron-phonon lifetimes. If commented, lifetimes are assumed constants. We can set the lattice temperature with BoseTemp and the number of modes entering in the simulation ElPhModes. In order to account of the temperature effects in a realistic ways the electron and hole damping GfnQP_Wv and GfnQP_Wc should be update for each temperature run. In most semiconductors, they are proportional to the electronic density of states. The second element of the array multiply the density of states by the given values. For instance, we could set:

run['GfnQP_Wv']   = [0.00,0.10,0.00]    # Constant damping valence
run['GfnQP_Wc']   = [0.00,0.10,0.00]    # Constant damping conduction

Below we show the carrier dynamics simulation including the electron-phonon dissipation of electrons and holes. We have made the example for two different temperatures. We only show the lifetimes of electrons and holes for 0 and 300 K. At each time step we show the mean value of the electron-phonon lifetime. We can observe that increases for larger temperature (see the Electron-phonon tutorial). Moreover, when the systems tends to the final state the mean EP lifetimes reachs a constant value.

_images/lifetimes.png

4.2 Electron-electron interaction

yambo -q p -s e

The inclusion of the electron-electron scattering needs the calculation of the electron-electron collisions files.

5. Use of Double-Grid in carrier dynamics simulation

The convergence of the results with the k-grid is a delicate issue in carrier dynamics simulations. In order to mitigate the simulation time we can use a double-grid. In our example we create the double-grid in three steps.

  1. We run a non-self-consistent simulation for a larger grid (4x4x4 in the silicon example). We find the results in the folder nscf-dg.
  2. We break the symmetries accordingly with our polarization field using the scripts. We indicate the output folder rt-dg, the prefix si and the polarization 100.
python break-symm.py -i nscf-dg -o rt-dg -p si -s 100
  1. We have created the script map-symm.py to map the coarse grid in the fine grid.
python map-symm.py -i rt-dg -o rt dg-4x4x4

The folder dg-4x4x4 is inside the rt folder. We will find a netCDF file ndb.Double_Grid. In order to tell yambo to read the Double-grid we have to indicate the folder name inside the -J option. In our example

yambo_rt -F 04_PUMP -J 'qssin,col-hxc,dg-4x4x4'

We can activate the double-grid in the python script rt_si.py by selecting:

job['DG'] = (True,'dg-4x4x4')

We can also check if yambo is reading correctly the double-grid in the report file. We have to find the lines:

[02.05] Double K-grid
  =====================

K-points             : 103
Bands                :  8

Electron-Phonon interaction (Si)

by A. Molina Sánchez

1. Ground State and non-self consistent calculation

Electron-phonon interaction calculations requires to obtain electronic states, phonon states and the interaciton between them. An extended study can be found in the Thesis of Elena Cannuccia.

Go to the tutorial folder and run the ground state calculation using the gs_si.py file:

python gs_si.py

The script will run a relaxation of the structure, read the optimized cell parameter and create a new input file that is used to run a self-consistent (scf) cycle and a non self-consistent (nscf) cycle using the charge density calculated on the previous run.

The self-consistent data are used to obtain the derivative of the potential. The non-self-consistent data are used, together with the potential derivative, for deriving the electron-phonon matrix elements.

_images/tutorial-el-ph_1.jpg

The script elph_pw_si.py calculates the electron-phonon matrix elements. It follows the indications of the flowchart, using the scf and nscf data. All the files used by QE are stored in the directory work. Finally, it transform the files from the QE format to the netCDF format used by yambo. It creates the folder elphon.

2. Electron-phonon calculations

The second step requires the script elph_qp_si.py. If the electron-phonon matrix elements have been successfully created and stored in elphon/SAVE we are ready to calculate the electron-phonon correction of the eigenvalues at several temperatures, or to examine the spectral function of each quasi-particle state. A detailed tutorial of the capabilities of the module electron-phonon of yambo is also available in the yambo website.

If we run:

python elph_qp_si.py -r

Yambo will calculate the quasi-particle correction and the spectral functions for the top of the valence band and the bottom of the conduction band (states 4 and 5). In order to plot the results we type:

python elph_qp_si.py -p

The QP correction due to the electron-phonon interaction are usually much smaller than those obtained with the GW approximation.

_images/elph-qp-correction.png

We can also plot the spectral function for a given state (n,k), i. e., the imaginary part of the Green’s function. This is a useful check of the validity of the QP approximation. A well-defined QP state will show a single-peak spectral function (or a clearly predominant one). A recent application in single-layer MoS2 is available here.

_images/elph-sf.png

We can play with more options by selecting the appropiate variables from the script elph_qp_si.py. For instance we can: (i) select only the Fan or Debye-Waller term, (ii) calculation on the on-mass-shell approximation, (iii) print the Eliashberg functions, etc.