The self-energy for a given electrode (Left or Right) is computed as Ʃ
L(E)=(H
CL - ES
CL) g(E) (H
LC - ES
LC), where g(E) is the surface Green's function of the semi-infinite (Left) electrode, and H
LC is the part of the Hamiltonian that couples the left electrode (L) with the central region (C), and likewise for the overlap matrix, S
LC.
If you are interested in the self-energy (matrix) you can get it like this:
sigma_L = calculateSelfEnergy(device_configuration, energy=0.0*eV, contribution=Left)
Notice, that you can also get access to the Hamiltonian, Green's functions and other quantities as described in this tutorial :
http://quantumwise.com/documents/tutorials/latest/LowLevelEntities/index.html/chap.intro.html#sect.intro.hs
The self-energy is a complex matrix. The real part gives rise to a shift of the molecular energy levels, while the imaginary part gives the broadening (finite life time) of the molecular energy levels.
Sometimes the coupling parameter (matrix) is referred to as Ґ
L(E) = i(Ʃ
tL(E) - Ʃ
L(E)), which is responsible for the energy broadening.
Since Ґ is a matrix, that depends on energy, the actual broadening of a particular molecular orbital should be obtained from the width of the spectral function A(E) = G
r(E)( Ґ
L(E) + Ґ
R(E)) G
a(E), where G
r/a(E) is the retarded/advanced Green's function, which can also be obtained (see link above).
