First of all compile QuantumEspresso and Yambo/Lumen. If you have problems in compilation please have a look to: compiling yambo.

The tutorial on linear response from real-time Schödinger equation is divided in 8 steps:

**1) DFT calculations**

In Lumen (and Yambo) all quantities are written in a basis of the Kohn-Sham(KS) eigenvalues and eigenvectors. In order to generate this basis we use a DFT code in plane wave as Abinit or QuantumEspresso(QE).

In this example we will consider a single later of hexagonal boron nitrite (hBN), the input files available here (QE_inputs or ABINIT_inputs). The first input file is a self-consistent(SCF) calculation that is used to generate the density of the system. The second input file is a non-self consistent(NSCF) calculation to diagonalize the KS Hamiltonian, that depends from the density of the first run, on for a given number of bands and k-points. Notice that parameters in the NSCF calculation determine the number of k-points and the maximum number of bands that can be used in Lumen. Run these calculation with the command:

for QuantumEspresso:

for Abinit:

Notice that in the NSCF file of QuantumEspresso we use the flag`force_symmorphic=.true.` to exclude the non-symmorphic symmetries that are not supported by Lumen(Yambo), in Abinit the same option is activated by the flag `symmorphi 0`.

**2) Import the wave-functions**

If you used QuantumEspresso go in the folder`bn.save`, in the ABINIT case all file are already in the main folder. Then import the wave-function with the command

for QuantumEspresso:

for Abinit:

**4) Setup**

Generate the setup input file with the command`yambo_nl -i -V RL -F setup.in`, then run `yambo_nl -F setup.in`. You can reduce the number of G-vectors in the setup in such a way to speed up calculations. I advise to reduce G-vector to 1000 (about 50% the initial ones).

**5) Reduce symmetries**

Since in real-time simulation we introduce a finite electric field in the Hamiltonian, the number of the symmetries of the original system is reduced due to the presence of this field. Using the tool`ypp -y` to generate the input file:

Set the external field in the`y` direction and uncomment the Time Reversal flag, as shown
in red above. Run `ypp` and it will create a new folder called `FixSymm` with the reduced symmetries wave-functions.

**6) Setup again**

Go in the`FixSymm` directory and run the setup again `yambo_nl -F ../setup.in`. Now everything is ready for the real-time simulations!

**7) Real-time dynamics**

In order to calculate linear-response in real-time we will perturb the system with a delta function in time external field. Use the command`yambo_nl -u -F input_lr.in` to generate the input:

We set the verbosity to "high" in such a way to print real-time output files.

We set the differential equation integrator to INVINT that is faster bul less accurate than the default(see PRB**88**, 235113). This integrator is ok in case of independent partcicles but I advise you to use CRANKNIC integrator when correlation effects are present. Now run `yambo_nl -F input_lr.in`

The code will produce different files:`o.polarization_F1` that contains the polarization, `o.external_potential_F1` the external field we used, and finally `r_optics_nloptics` a report with all information about the simulation.
If you plot the third column of `o.polarization_F1` versus the first one (time variable) you will get the time-dependent polarization along the y direction:

**8) Analyze the results**

Now we can use`ypp_nl -u` to analyze the results:

`ypp_nl` and obtain the following files: the dielectric constant along the field direction `o.YPP-eps_along_E`, the EELS along the same direction `o.YPP-eels_along_E`, and the damped polarization `o.YPP-damped_polarization`.

Now we can plot the dielectric constant and compare it with linear response:

The input for the linear response can be downloaded here. Notice that in a real-time simulation we obtain directly the \( \chi(\omega) = \frac{P(\omega)}{E(\omega)} \) that is realted to the dielectric constant from the relation $$\epsilon(\omega) = 1 + 4 \pi \chi(\omega) $$

The tutorial on linear response from real-time Schödinger equation is divided in 8 steps:

In Lumen (and Yambo) all quantities are written in a basis of the Kohn-Sham(KS) eigenvalues and eigenvectors. In order to generate this basis we use a DFT code in plane wave as Abinit or QuantumEspresso(QE).

In this example we will consider a single later of hexagonal boron nitrite (hBN), the input files available here (QE_inputs or ABINIT_inputs). The first input file is a self-consistent(SCF) calculation that is used to generate the density of the system. The second input file is a non-self consistent(NSCF) calculation to diagonalize the KS Hamiltonian, that depends from the density of the first run, on for a given number of bands and k-points. Notice that parameters in the NSCF calculation determine the number of k-points and the maximum number of bands that can be used in Lumen. Run these calculation with the command:

for QuantumEspresso:

pw.x -inp BNsheet.scf.in > output_scf pw.x -inp BNsheet.nscf.in > output_nscf

for Abinit:

abinit < hBN.files > output_hBN

Notice that in the NSCF file of QuantumEspresso we use the flag

If you used QuantumEspresso go in the folder

for QuantumEspresso:

p2y -F data-file.xml

for Abinit:

a2y -F hbn_out_DS2_KSS

Generate the setup input file with the command

Since in real-time simulation we introduce a finite electric field in the Hamiltonian, the number of the symmetries of the original system is reduced due to the presence of this field. Using the tool

fixsyms # [R] Reduce Symmetries % Efield1 0.00 | 1.00 | 0.00 | # First external Electric Field % % Efield2 0.00 | 0.00 | 0.00 | # Additional external Electric Field % #RmAllSymm # Remove all symmetries RmTimeRev # Remove Time Reversal

Set the external field in the

Go in the

In order to calculate linear-response in real-time we will perturb the system with a delta function in time external field. Use the command

nlinear # [R NL] Non-linear optics NL_Threads= 1 # [OPENMP/NL] Number of threads for nl-optics % NLBands 3 | 6 | # [NL] Bands % NLstep= 0.0100 fs # [NL] Real Time step length NLtime=55.000000 fs # [NL] Simulation Time NLverbosity= "high" # [NL] Verbosity level (low | high) NLintegrator= "INVINT" # [NL] Integrator ("EULEREXP/RK4/RK2EXP/HEUN/INVINT/CRANKNIC") NLCorrelation= "IPA" # [NL] Correlation ("IPA/HARTREE/TDDFT/LRC/JGM/SEX/HF") NLLrcAlpha= 0.000000 # [NL] Long Range Correction % NLEnRange 0.200000 | 8.000000 | eV # [NL] Energy range % NLEnSteps= 1 # [NL] Energy steps NLDamping= 0.000000 eV # [NL] Damping % ExtF_Dir 0.000000 | 1.000000 | 0.000000 | # [NL ExtF] Versor % ExtF_kind= "DELTA" # [NL ExtF] Kind(SIN|SOFTSIN|RES|ANTIRES|GAUSS|DELTA|QSSIN)The standard input of Lumen is thought for non-linear response so we have to change some parameters in order to calculate the linear response. Set the field direction along y, the field type to DELTA, the length of the simulation to 55 fs, number of bands from 3 to 6 dephasing to zero and the number of energy steps to one, as shown above in red.

We set the verbosity to "high" in such a way to print real-time output files.

We set the differential equation integrator to INVINT that is faster bul less accurate than the default(see PRB

The code will produce different files:

Now we can use

nonlinear # [R] NonLinear Optics Post-Processing Xorder= 1 # Max order of the response functions % TimeRange -1.000000 |-1.000000 | fs # Time-window where processing is done % ETStpsRt= 200 # Total Energy steps % EnRngeRt 0.00000 | 10.00000 | eV # Energy range % DampMode= "LORENTZIAN" # Damping type ( NONE | LORENTZIAN | GAUSSIAN ) DampFactor= 0.10000 eV # Damping parameterwhere we set a lorentzian smearing corresponding to 0.1 eV. Notice that due to the finite time of our simulation a smearing is always necessary to Fourier transform the result. Then we run

Now we can plot the dielectric constant and compare it with linear response:

The input for the linear response can be downloaded here. Notice that in a real-time simulation we obtain directly the \( \chi(\omega) = \frac{P(\omega)}{E(\omega)} \) that is realted to the dielectric constant from the relation $$\epsilon(\omega) = 1 + 4 \pi \chi(\omega) $$