Aim:
I want to obtain a reasonably accurate ionization energy (or work function in other terms) for a 2-dimensional hBN monolayer. The exact material does not matter much respective to the question, the point is it being a 2D periodic material.
Method:
The GW model, in general, gives a pretty accurate ionization potential. For an isolated molecule, the SCM code (in the AMS package) works, I've tested it for isolated h-passivated hBN nanoparticles. However, the code does not implement GW for periodic systems, and I couldn't reach a convergent territories by increasing the nanoparticle size before running out of computer resources. Therefore, I turned to Yambo.
Doubt:
If I'd want to obtain $I_p$ (or $W$) from a Quantum Espresso DFT calculation, I would have to calculate the vacuum energy (from the average potential), and subtract this from the HOMO value. On the other hand, the SCM documentation (specifically, the G0W0 tutorial) states that the resulting HOMO energy from a G0W0 calculation, $E_{GW}^{HOMO}$, is the negative of the ionization potential,$I_p$:
$$
I_p = - E_{G_0W_0}^{HOMO}
$$
However, they give no reference to this statement, and I couldn't find anything useful.
The various tutorials of Yambo (e.g. this) on G0W0 only use the GW energies to obtain the band gap value. This is a relative energy between two bands, so tells me nothing if the absolute values are WRT vacuum.
Is it a fundamental property of G0W0 that $E_{GW}^{HOMO} = -I_p$, or is this specific to the SCM code?
If no, is it at least true for Yambo as well?