I am trying to do a G0W0 calculation of a molecule with yambo 3.4.1 (using Coulomb cutoff etc. so the cell is rather large)
and I am trying to converge the X matrix size.
Below I have summarized the most important results (run time refers only to the time used to calculate X).
NGsBlkXp [Ry] = 1 2 3 4
NGsBlkXp [RL] = 921 2553 4713 7237
run time
Gap [eV] = 6.921 6.848 6.870 6.869
I have two questions:
1. A very basic question about plane waves: How should one choose the grid of NGsBlkXp values to check for convergence?
If I choose my points in a way that their relative distance decreases further and further (in the 'relevant metric'), the results might appear to be near convergence although they aren't. Is the 'relevant metric' here the number of plane waves?
I.e. given that E \propto k^2, should I rather use a grid of NGsBlkXp = 1^2, 1.5^2, 2^2, 2.5^2, ... Ry ?
2. Are there any tricks to reduce the run time of the X calculation?
I guess, the dramatic increase in run time can be explained by the cost of the matrix inversion being proportional to nGsBlkX^3 (?).
So, if I cannot reduce the cell size, is there anything I can do?
Best,
Leopold
P.S.: Although the gap value seems to be well converged, some orbital energies near Fermi still change by 20meV from 3Ry to 4Ry.