Question about pbc ewald part. about ferminet HOT 2 CLOSED

Hi Xiang, thanks for your interest in the release and for taking a careful look at the code!

You are absolutely correct that if we were applying a minimum image convention, that code would be wrong. However, these lines are only there because as walkers move far from the origin, they move outside the region of physical space where we are actually taking the Ewald sum. We only need to move the walkers 'close' (by some definition related to the truncation length) to the origin, and so an electron occasionally being one image over from the minimum image does not cause a problem.

To lay your mind at ease: I have calculated the potential of two electrons separated as you suggested by a distance of 0.6 units with/without your suggested edit (here I am using NumPy only, no JAX, and double precision):

without edit: -2.75527802498627
with edit: -2.7552780249862696

(simple cubic lattice, lattice constant 1 bohr, no atomic potential, include_heg_background=True)

I'm not sure this -needs- to be fixed, however, it cannot hurt and might save someone anxiety about the same problem in future. I would like to lay any readers' minds to rest thought that this does not cause the code to give 'wrong answers' as far as I can tell.

from ferminet.

Dear Gcassella,

Thanks for the quick response. I agree that the difference is quite small and won't affect the correctness of present work. I just worry that this difference might lead to some problems at extremely special cases and that's why I raise the issue.

from ferminet.