Real-time dynamics with external field read from file
In this tutorial we will see how to generate an external field in a format compatible with Lumen and then use it to run real-time simulation (see for instance Ref. [1].
We will also prove a simple python script to do it, that you can easily modify to create your own external field.
The tutorial is divided in three sections: 1) generate external field with YamboPy; 2) generate external field with ypp_rt; 3) use the external field in the real-time calculations.
Time-step choice
When generating an external field, care must be taken in choosing the time step to ensure compatibility with the integrator.
- EULER and INVINT are single-time integrators; therefore, your time step must be equal to the one used in the simulation.
- CRANKNIC, RK2EXP and RK2 are two-time integrators; the time step should be half the one used in the simulation.
- RK4 and RK4EXP are four-time integrators that requires 1/2 and 1/3 of the time-step, therefore you have to divide the time-step by 6 to be compatible with these integrators.
anyway even if you make a mistake the code check the time-step of the external field before start simulations.
Generate external field with YamboPy
In YamboPy you can find a tutorial to generate external field in the folder yambopy/tutorial/external_field/generate_field.pyYou can define your external field in this way:
def gauss_field(t_steps):
# In this example we generate a guassian external field
sigma=5.0*fs2aut # Field width
T_0 =3.0*sigma
Expf =np.exp(-(t_steps-T_0)**2/(2.0*sigma**2) )
a_t =sigma*np.sqrt(math.pi/2.0)*(special.erf((t_steps-T_0)/(sigma*np.sqrt(2.0)))+1.0)
ap_t =Expf
app_t=-Expf*(t_steps-T_0)/sigma**2
return a_t,ap_t,app_t
Just a function that generate a(t), a'(t), a(t) that will be then read from Yambo_rt/Yambo_nl. You can add you function to generate_field.pyscritp or use the two that are already implemented
SIN and GAUSS fields. Notice that the electric field correspond to E(t) = -E_amp a'(t).
The field amplitude (intensity) and versor are not required because it will be read from the Yambo input
Nota bene: only linear polarized external field can generated with YamboPY (to be generlized)
Generate external field with ypp_rt
With ypp_rt one can generate all external fields that implemented in the Yambo code in the file src/modules/mod_fields.F. This can be usefull to visualize external fields used in Yambo without the need of running a real simulation. However if you want to add a new external field you have to code it in the file src/modules/mod_fields.F. The command to generate external field on file is: ypp_rt -rtplot f. This will generate the following input file:
TDplots # [R] TD observables plot RTtime # [R] Post-Processing kind: function of time RTfields # [R] Analize time-dependent fields TimeStep= 0.010000 fs # Time step % TimeRange 0.000000 |100.000000 | fs # Time-window where processing is done % % EnRngeRt 0.00000 | 20.00000 | eV # Energy range % ETStpsRt= 200 # Total Energy steps ChirpFac= 0.000000 fs # Prefactor for linear chirping in frequency space Field1_Freq= 2.000000 eV # [RT Field1] Frequency Field1_Int= 1000.000000 kWLm2 # [RT Field1] Intensity Field1_Width= 0.000000 fs # [RT Field1] Width Field1_kind= "SIN" # [RT Field1] Kind(SIN|COS|RES|ANTIRES|GAUSS|DELTA|QSSIN) Field1_pol= "linear" # [RT Field1] Pol(linear|circular) % Field1_Dir 1.000000 | 0.000000 | 0.000000 | # [RT Field1] Versor % Field1_Tstart= 0.010000 fs # [RT Field1] Initial Time
In the input file you have to specify a series of parameters, in red: the time step (TimeSpep), the time range (TimeRange), the field kind (Field1_kind), its parameters (Field1_Width, Field1_Freq, etc...) and finally the starting time (Field1_Tstart), usually the first step of the dynamics.
Notice that Field direction (Field1_Dir) field intensity (Field1_Int) are not used in the external field generation but read from Yambo input.
Then you run ypp_rt with this input and it will produce a series of files: YPP_EXTFIELD1_P1.time, YPP_EXTFIELD1_P2.time, YPP_EXTFIELD1_P1.freq, YPP_EXTFIELD1_P2.freq.
These files contain the external field and its Fourier transform. In the next section we will see how to use them in real-time dynamics.
Use generated external field with yambo_rt/yambo_nl
In yambo_rt/yambo_nl you can tell to the code to read an external field from file. For example in yambo_nl you generate your input wil the command yambo_nl -u p
nloptics # [R] Non-linear spectroscopy % NLBands 7 | 10 | # [NL] Bands range % NLverbosity= "high" # [NL] Verbosity level (low | high) NLtime=120.000000 fs # [NL] Simulation Time NLintegrator= "INVINT" # [NL] Integrator ("EULEREXP/RK2/RK4/RK2EXP/HEUN/INVINT/CRANKNIC") NLCorrelation= "IPA" # [NL] Correlation ("IPA/HARTREE/TDDFT/LRC/LRW/JGM/SEX/LSEX/LHF") NLLrcAlpha= 0.000000 # [NL] Long Range Correction NLDamping= 0.200000 eV # [NL] Damping (or dephasing) RADLifeTime=-1.000000 fs # [RT] Radiative life-time (if negative RADLifeTime=Phase_LifeTime) #EvalCurrent # [NL] Evaluate the current HARRLvcs= 11529 RL # [HA] Hartree RL components EXXRLvcs= 11529 RL # [XX] Exchange RL components CORRLvcs= 11529 RL # [GW] Correlation RL components Field1_Freq= 0.100000 eV # [RT Field1] Frequency Field1_Int= 1000.00 kWLm2 # [RT Field1] Intensity Field1_Width= 0.000000 fs # [RT Field1] Width Field1_kind= "FROM_FILE ./YPP_EXTFIELD1_P1.time" # [RT Field1] Kind(SIN|COS|RES|ANTIRES|GAUSS|DELTA|QSSIN) Field1_pol= "linear" # [RT Field1] Pol(linear|circular) % Field1_Dir 1.000000 | 0.000000 | 0.000000 | # [RT Field1] Versor %
You have to specify the file kind FROM_FILE and give the path of the file containing the external field. Do not forget also to use the same NLtime and in the ypp and yambo inputs. Notice that field intensity and field versor is specified in the Yambo input only.
Notice that NLtime in the simulation can be larger then the one used in the external field generation, the code will assume that field that after a given time the field is zero.
References
- ↑ D. Sangalli, Excitons and carriers in transient absorption and time-resolved ARPES spectroscopy: An ab initio approach, Phys. Rev. Materials 5, 083803, (2021).
