Return the external potential in the dual basis of arXiv:1706.00023.

The external potential resulting from electrons interacting with nuclei in the plane wave dual basis. Note that a cos term is used which is strictly only equivalent under aliasing in odd grids, and amounts to the addition of an extra term to make the diagonals real on even grids. This approximation is not expected to be significant and allows for use of even and odd grids on an even footing.

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. 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 dual basis operator.