I have a question about FFTGvecs to expand the wavefunctions. I found if I used a larger cutoff (20Ry around 11300 plane waves) to do the ground state calculation then I cut the plane waves to 5000 (FFTGvecs=5000) for the expansion of the wavefunctions to compute BSE spectrum,

**it's better**than if I directly use around 10Ry(5600 plane waves) to do ground state calculation then compute the BSE spectrum when I compare these two cases with 20Ry calculation without cuting plane waves for expansion of the wavefunction in BSE calcution. I thought if you use the same cutoff to do ground state calculation then compute the spectra is equivalent to cut the plane waves to do BSE calculation. How to understand this?

yuan ping

University of California, Davis