questions about SHG of InSe

[Dean]

Dear developers,
I am reading the paper "Second-harmonic generation in single-layer monochalcogenides: A response from first-principles real-time simulations" (PHYSICAL REVIEW MATERIALS 3, 074003 (2019)). I am very interested in the calculations of SHG of 2D materials and wnat to learn it.
So, I did some calculations of InSe monolayer, but something goes worng. Can you give me some help?
Here is my yambo_nl input file:
FFTGvecs= 21125 RL # [FFT] Plane-waves
% NLBands
16 |23 | # [NL] Bands
%
NLstep= 0.02 fs # [NL] Real Time step length
NLtime=75.000000 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")
NLLrcAlpha= 0.000000 # [NL] Long Range Correction
% NLEnRange
0.300000 | 5.000000 | eV # [NL] Energy range
%
NLEnSteps= 80 # [NL] Energy steps
NLDamping= 0.100000 eV # [NL] Damping
% ExtF_Dir
1.000000 | 0.000000 | 0.000000 | # [NL ExtF] Versor
%
ExtF_FrStep= 0.000000 eV # [NL ExtF] Frequency step
ExtF_Int= 500. kWLm2 # [NL ExtF] Intensity
ExtF_Damp= 2.000000 fs # [NL ExtF] FWHM
ExtF_kind= "SOFTSIN" # [NL ExtF] Kind(SIN|SOFTSIN|RES|ANTIRES|GAUSS|DELTA|QSSIN)
ExtF_Tstart= 0.0100 fs # [NL ExtF] Initial Time
GfnQPdb= "none" # [EXTQP G] Database
GfnQP_N= 1 # [EXTQP G] Interpolation neighbours
% GfnQP_E
1.330000 | 1.000000 | 1.000000 | # [EXTQP G] E parameters (c/v) eV|adim|adim
%

And ypp_nl input file:
nonlinear # [R] NonLinear Optics Post-Processing
Xorder=4 # Max order of the response functions
% TimeRange
34.00 | 75.00 | fs # Time-window where processing is done
%
ETStpsRt=200 # Total Energy steps
% EnRngeRt
0.30 | 5.00 | eV # Energy range
%
DampMode= "NONE" # Damping type ( NONE | LORENTZIAN | GAUSSIAN )
DampFactor= 0.100000 eV # Damping parameter

My question is : 1. Are the input files right?
2. How to deal with the output file "o-2D.YPP-X_probe_order_2" in order to get the SHG coefficients of InSe?
Thanks in advance.

[claudio]

Dear Dr. Yimin Ding

your input files seem correct to me. I will only change the NLstep= 0.01 fs

what problem did he have?

Regarding the file o-2D.YPP-X_probe_order_2 it contains the X2 coefficients. The X^2_{xxy},
because you put the incoming field in x and measure the non-linear response in y.


In particular, the ones plotted in the paper are the 5 and 6 columns.
Remember that you have to rescale them:
1) fist from m/V to the CGS system, divide for 2.38721e-09
2) you have to rescale for the box size respect to the width of the material, for example,
if you have simulation box 35 a.u. high and your material width is 15.042219756 a.u.
you rescale for 35.0/15.042219756

let me know if it is clear
best
Claudio

[Dean]

Dear Dr. Claudio,
Thanks for your reply.
My plot of X2 coefficients shows that the peak shifts to higher energy range. See the attachments please.
Look forward to your reply.

[claudio]

Dear Dr. Yimin Ding

I agree with you, but in your input, you introduced a shift of 1.33 eV

% GfnQP_E
1.330000 | 1.000000 | 1.000000 | # [EXTQP G] E parameters (c/v) eV|adim|adim
%

while in the IPA plot of my paper this is not present.
Try to use 0 shift and a mearing of 0.2 eV, you will get the same plot in IPA.

let me know
Claudio

[Dean]

Dear Dr. Claudio,
Thanks for your reply.
I still have the following questions:
1.Why the Xorder in ypp.in is set as 4 not 2?
2.The smearing you said is "NLDamping= 0.100000 eV" in yambo.in or "DampFactor= 0.100000 eV" in ypp.in?
3. I initially want to repeat the results of G0W0+BSE. How to do that?
4. The Tutorials of non-linear calculations will be put on the new official website of yambo or wiki website?
Look forward to your reply.

[claudio]

Dear Dr. Yimin Ding

here the reply to your questions:

>1.Why the Xorder in ypp.in is set as 4 not 2?

the non-linear response is extract by fitting the time dependent polarization, we think it is better to use more coefficient
for the fit up to 4 I will say. See Eq. 26, 27 and 28 of https://arxiv.org/pdf/1309.4012.pdf.
You can test if putting only 2 gives you the same result.

>2.The smearing you said is "NLDamping= 0.100000 eV" in yambo.in or "DampFactor= 0.100000 eV" in ypp.in?

The DampFactor in ypp.in is valid only in linear optics, and actived if you select a DampMode different from "NONE"
I will add a comment in the tutorial on this point.

>3. I initially want to repeat the results of G0W0+BSE. How to do that?

Follow the step of the turorial on BSE
http://www.attaccalite.com/lumen/real_time_bse.html
and calculate the non-linear response.

>4. The Tutorials of non-linear calculations will be put on the new official website of yambo or wiki website?

yes, soon.

best regards
Claudio Attaccalite

[Dean]

Dear Dr. Claudio,
Thanks very much.