Zirconium metal must essentially be hafnium free (<100 ppm Hf) to be suitable for use in nuclear reactors. However, while zirconium and hafnium always occur together in nature, they have very similar chemical properties, which complicate their separation. An alternative separation process to the widely used liquid-liquid extraction and extractive distillation processes could be the fractional crystallization of K₂Zr(Hf)F₆, which can be produced by the plasma dissociation of zircon and subsequent chemical processing. However, the formation of solid solutions by lattice substitution will greatly hamper the separation efficiency in a fractional crystallization process. A disturbance in the crystal lattice of K₂ZrF₆, caused by the inclusion of Hf-impurities, will result in a change in lattice energy. Therefore the purity of crystallized K₂ZrF₆ is determined by the distribution coefficient of the Hf-specie, i.e. its tendency to be incorporated into the K₂ZrF₆ crystal lattice. In this study we propose a method for determining the influence of Hf-impurities on the Gibbs free energy of the crystal lattice of K₂ZrF₆ using molecular modelling software, whereby the thermodynamic distribution coefficient of the Hf-impurity can be predicted.