Return the external potential operator in plane wave basis.

The external potential resulting from electrons interacting with nuclei. It is defined here as the Fourier transform of the dual basis Hamiltonian such that is spectrally equivalent in the case of both even and odd grids. Otherwise, the two differ in the case of even grids.

grid (Grid): The discretization to use.
geometry A list of tuples giving the coordinates of each atom. example is [('H', (0, 0, 0)), ('H', (0, 0, 0.7414))]. Distances in atomic units. Use atomic symbols to specify atoms.
spinless Bool, whether to use the spinless model or not. e_cutoff (float): Energy cutoff. non_periodic (bool): If the system is non-periodic, default to False. period_cutoff (float): Period cutoff, default to grid.volume_scale() ** (1. / grid.dimensions)

FermionOperator The plane wave operator.