Absorption spectrum and exciton strengths

[yunho.ahn]

Dear Yambo community,

I've been calculating the optical spectrum of an organic charge transfer complex materials using GW-BSE by following tutorials.
I wanted to overlay the exciton strengths onto the calculated absorption spectrum to present individual exciton contributions shape the absorption spectrum.

One thing that I'm confused is that, as far as I understand, the exciton strength is the linear combination of the square of the dipole transition matrix elements between electron-hole pairs and the imaginary part of the dielectric function is derived from them, shouldn't the exciton strength be equal to or less than the imaginary part of epsilon? even though it's normalized.

But I got the exciton strength, which is larger than the corresponding imaginary epsilon (especially the first one). Could you help me understand why this might be the case?

Thank you in advance for your insights.

Best,
Yunho

[Daniele Varsano]

Dear Yunho,

One thing that I'm confused is that, as far as I understand, the exciton strength is the linear combination of the square of the dipole transition matrix elements between electron-hole pairs and the imaginary part of the dielectric function is derived from them

Correct! the dipoles are weighted with the corresponding BSE eigenvectors.

But I got the exciton strength, which is larger than the corresponding imaginary epsilon (especially the first one). Could you help me understand why this might be the case?

Please note the following:
1. The strength is the area beneath each peak of Im(eps) and not its height. The eps is obtained as convolution of Lorentzian function centered on each excitation, its intensity depends on the adopted smearing (broadening).

2. The strength in the output are normalized such that the largest value assumes the value 1. (The renormalization is reported in the exc file (Maximum Residual Value).

Best,

Daniele