VI. ACKNOWLEDGEMENT S. Seyedraoufi, C.H. Gørbitz, and O. Nilsen are acknowledged for valuable discussions. The computations of this work were carried out on UNINETT Sigma2 high performance computing resources (grant NN9650K). This work is supported by the Research Council of Norway as a part of the Young Research Talent project FOX (302362). The crystal structures of the plastic ionic molecular crystals in this work were retrieved from the Cambridge Structural Database . Crystal structure figures have been made with the VESTA software .