• Different nature of sources and targets: Traditional libraries are often meant for symmetric interactions in which the nature of the sources and targets is similar. In ddX, the sources are the multipolar distributions at the center of the spheres, while the targets are the integration points on the surface of the spheres. This has two main consequences: i) a particle-to-multipole step (P2M) is missing, whereas a local-to-particle (L2P) step takes care of evaluating the electrostatic properties at the integration points; ii) the whole FMM scheme is not symmetric, as there is an asymmetry in the targets and sources despite the kernel being symmetric.