Bugfix: FFT for 2D periodic systems
Description
The 3D-2D kernel was missing a cutoff. This was leading to imprecise forces. For hBN, this was leading to 1e-3 forces, whereas with the cutoff, the forces are 1e-12, as for the 3D-3D case.
Always compute the force from the local potential with real number.
News snippet
Minor optiminization and more precise forces for 2D periodic systems using FFT.
Checklist
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I have checked that my code follows the Octopus coding standards -
I have added tests for all the new features added in this request.
Edited by Nicolas Tancogne-Dejean