Within the framework of ddX, the solute molecule is entirely described by the cavity Ω and the charge distribution ρ. As it is customary in continuum solvation models, it is assumed that the charge ρ is supported in Ω, i.e. ρ(x) = 0 for x ∈ Ω C . As we will see in the following, different models and thus different codes have different representations for the charge distribution ρ. Therefore, ρ can be represented either by a sum of (partial) point-charges or, more generally, a sum of multipoles for classical force-field models, or a sum of point-charges (nuclear charges) in combination with the electronic density ρ el for QM-based models.