How to determine the size of the FFTGvecs?

[gaohan]

Dear Developers,
I know the FFTGvecs is the number of G-vectors or energy cutoff for expanding wavefunctions in transition matrix elements and FFT operations, and adjusting that can regulate the memory of calculation. But I wonder how to determine it?
For example, I do the DFT calculation using 450eV for the first step. Is that meant to be that I should set the FFTGvecs to 450 eV?
The default value is 15529 RL when I generate the yambo input file.
Looking forward to your reply.

[Daniele Varsano]

Dear Han Gao,

if you are not experiencing memory problem you can leave the default value.
If you need to lower it, a reasonable criteria is to look at the orthonormalisation of the wfs which is reported when calculating e.g. dipoles or HF self-energy:
e.g.

Code: Select all

[WF-Oscillators/G space loader] Normalization (few states)  min/max  :0.260E-09  1.00

and verify that it does not differ from 0,1 which means <n|n'>=delta_nn'

Best,
Daniele