Dear Andreas,
Fortunately the cluster's admin was able to repair the cluster
OK, that's' better, in mac it is not rare to encounter problems.
1) Concerning the option WRbsWF - has the explanation just a spelling mistake and "FWs" means "WFs = WaveFunctions" ?
Yes it is a spelling mistake.
2) IN g-space you can calculate the response for finite q, so you can control the QpntsRXd variable. In transition space only the q-->0 limit is implemented. The Hamiltonian is divided in blocks, this is because you have many fragments.
the speed of the gspace calculations (which lasted only some seconds) - whereas the "yambo -o b -k alda" calculations take several days
Well it depends on what you ask. To have an equivalent calculation in g-space you should consider to set NGsBlkXd= 5961, this would mean to invert a matrix six thousand by six thousand for each energy step (by default 100 times), I doubt that it will be a few seconds calculation. In transition space you diagonalize the excitonic matrix (that could be bigger) just once.
The number of reciprocal lattice vectors (5961) is very high - I would reduce it to around 1000 + some convergence tests. Is this comparable to the case where I reduce the energy cutoff in the PWscf calculation?
Well it is not so high, anyway sure, you can try to reduce it by doing some convergence tests. Usually you need less vectors than the energy cutoff you used for the wfs in the ground state calculations. Please note, that this number (5961) are the G-vectors entering in the coulomb part. For the ALDA kernel you have automatically all the Gvecotrs included as in transition space this part is calculated in real space (while in G-space calculation, this is not true anymore).
I would have a look on the bandstructure and would take only 10 bands below and 10 bands above the fermi level in "BSEBands" into account.
Yes you have a huge number of bands, which means yout can have a very large excitonic matrix (it is called excitonic matrix in BSE, and not in ALDA, but ok, the matrix which the eigenvalues gives you the excitation energies). The dimension of the matrix is Nv x Nc x Nk x Nsym. The number of bands entering in the calculations, as usual is another variable where the convergence has to be checked, it is not a bad idea to start with few bands and adding them progressively until convergence.
I reduce the number of k-point sampling in the brillouin zone in the PWscf input file which should reduce the number of fragments.
K-point is another variable, the convergence has to be tested. As said before, a very large sampling will imply a larger matrix, so large number of elements to be built and time to diagonalize it.
Best,
Daniele