  Ferroelectricity is commonly understood in terms of dynamical charges, which
represent the dipole moments generated by atomic displacements or the forces
induced by electric fields. In ferroelectrics with a high degree of symmetry,
the dynamical charges are typically symmetric tensors, and can be visualized as
ellipsoids. In van der Waals (vdW) materials which break centrosymmetry, a new
type of ferroelectricity arises which differs greatly from conventional
ferroelectrics. The polarization is purely electronic, arising from an
interlayer charge transfer, and most of the polarization generated is
perpendicular to atomic motion. We show that the unconventional properties of
vdW ferroelectrics are manifested in their dynamical charges, which exhibit
spatial modulation and intrinsic asymmetry. Dynamical charges in vdW
ferroelectrics, and more generally, any strongly anisotropic ferroelectric, can
be visualized as deformable, non-ideal ellipsoids dependent on the atomic
configuration. Furthermore, we show that, due to the mixed electrostatic
boundary conditions employed for two-dimensional materials, non-diagonal
dynamical charges in 2D materials are always asymmetric.
