Dear Pavel,

Do I understand correctly, that your major concern is to reproduce the experimental band gap in Si?

There are many things to mention prior going into technical details.

What is the value you're aiming at? There are many experimental as well as the theoretical results.

You can find in the literature the hole spectrum of G0W0 band gaps depending on the input structure and implementation.

Correct me if I'm wrong, the value you have reported before is the indirect band gap between X1(0,0.3,0.3)(CBM) [in the reciprocal unitcell units] and Gamma(VBM). This quite close to the X* point. However, since you're doing optics, I'd rather recommend you to look at the convergence of the direct band gap instead.

Before playing with the basis sets and fighting for the ultimate numerical convergence, I'd still recommend you to check the parameters which remained hidden (default). What about to expand the <gw> element:

<gw

taskname="g0w0"

ngridq="6 6 6"

nempty="300"

ibgw="1" nbgw="22"

>

<mixbasis

lmaxmb="3"

epsmb="1.0d-4"

gmb="1.0d0"

></mixbasis>

<barecoul

barcevtol="0.1"

></barecoul>

<freqgrid

nomeg="32"

freqmax="1.d0"

></freqgrid>

<selfenergy

actype="pade"

><selfenergy/>

<scrcoul

scrtype="rpa"

></scrcoul>

</gw>

The grid 6x6x6 should already be quite good, especially for tests. The product basis parameter could be an issue for some materials, but not in 'simple' Si. The number of frequencies is rather important and should not be overlooked.

In the posts above, you were correctly mentioning that the total number of the available empty states is restricted by the chosen in <groundstate> rgkmax parameter (Gmax). This number you should be able to see in KPOINT.OUT file, last column. So to reach high values of nempty, one has to modify rgkmax and recalculate groundstate cycle.

Thus, in your case for rgkmax=8, 300 empty states should already exceed the maximum number of the augmented plane waves.

You have also correctly mentioned that to reach numerically converged results it's often not enough to operate with the well converged groundstate (starting-point dependence), the number of states and the size of k/q-grids. One should extra take care about the complimentary basis-set incompleteness error. Similarly to pseudopotential method, for the excited state calculations one needs to employ specially optimized LAPW+LO basis (choose another species file). However, it has been shown a few times, that the basis set incompleteness is rather small for Si. So the reason for the inconsistent results one should look somewhere else.

The my suggestion is to modify the gw element and repeat calculations. The values you should ~1.21 eV [Gamma-X] and 3.21 eV [Gamma-Gamma] (depends on the lattice parameters).

Please let me know about your findings.

With best regards,

D. Nabok

(the exciting developers team)