  Like many other physical quantities, the optical force can be expanded using
multipole expansion, which has been done in [Nat. Photon. 5, 531], up to
electric octupole order. However, in that study, the existence of radiation
multipoles were pre-assumed, and the role of the fundamental building units,
charges, are not evident. Here, we derive the same multipolar expression of
optical force by treating the particles as a collection of point charges or
point dipoles, which results in more transparent physics and mathematics.
