BSE output: dark singlet excitons

[DavidPolito93]

Dear all,

I have performed BSE calculations on top of the G0W0 quasiparticle calculation.
The starting DFT point is nonmagnetic and I have kept BSENGexx value different from zero: if I am not wrong the solutions of the BSE equation should be singlet states then.

I have checked convergence of the results with respect to the various input parameters and to the k-grid in the TD approximation of the BSE Hamiltonian.
Then, I have performed a 'coupling' calculation with also the flag WehCpl active: in this way I should have performed a full BSE calculation.
Is it safe to use the same converged parameters in the full coupling version or is there something elese that should be checked again?

Assuming that everything is converged, I have two questions regarding the results:

1) I have sorted the excitons and looked at the o-***.exc_qpt1_E_sorted file: in the TDA the E [ev] column shows only positive energies, while in the full coupling computation it also shows negative energies. Is it due to the symmetry of the full BSE Hamiltonian?

2) Sorting the excitons and looking at their dipole strengths in the o-***.exc_qpt1_E_sorted file I have found that the first excitons are:

# E [ev] Strength Index
#
-1.134847044945 0.999992907047 4.000000000000
-.9094768167 0.1157104812E-7 2.000000000
-.9070294499 0.8592073719E-8 3.000000000
-.791058302 0.364091778E-10 1.00000000
0.791060328 0.403235917E-10 5.00000000
0.9070384502 0.1002118655E-7 7.000000000
0.9095062017 0.1143503514E-7 6.000000000
1.134868860245 1.000000000000 8.000000000000

Can I say that excitons whose strength is of the order E-7 and E-10 are dark? (in this case their index would be 5, 6 and 7 for positive energies)

Thank you in advance for your answer!

Best,

Davide Romanin
-----------------------------------------------------
Post-doctoral fellow
Institut des NanoSciences de Paris
Sorbonne Université, CNRS
4 place Jussieu,
75252 PARIS Cedex 05
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[DavidPolito93]

By the way,

sorry if I wrote in the wrong section... I thought I clicked on linear response

Davide

[Daniele Varsano]

Dear Davide,

1. Parameters used for TD approximation should be ok also when adding the coupling
2. When adding the coupling you are mixing resonant and resonant part, this is the reason why you have also negative eigenvalues and they are nearly the opposite of the positive ones when the effect of the coupling is small
3. I would say yes, there are 7 to 10 order of magnitudes with respect the most bright.

Best,
Daniele

[DavidPolito93]

Dear Daniele,

Thank you for your answer

Another question: when post processing it happens for example that

State 6 Merged with states 7 -> 8

Does it mean that states 6 and 7 are practically undistinguishable from state 8?

Davide
-----------------------------------------------------
Post-doctoral fellow
Institut des NanoSciences de Paris
Sorbonne Université, CNRS
4 place Jussieu,
75252 PARIS Cedex 05
------------------------------------------------------

[Daniele Varsano]

Dear Davide,
by default excited states that differs less than a threshold are merged as degenerates.
You can control the threshold via the variable:

Code: Select all

Degen_Step

Setting to 0.0eV you will have no merging of excitations.

Best,
Daniele