All ground states featured on the K-CO convex hull shown in Figure are based on differently charged rhodizonate anions (Figure ). Rhodizonates have a remarkable ability to hold different charges, and while a formal charge of -2 is most common in experimental structures, also -4 and -3 have been inferred. One of these, the Fddd phase of K2C6O6, is the only pure K-oxocarbon material in the Cambridge structural database (Figure ). Because of its size (28 atoms in the primitive cell) the Fddd phase could not be identified by our structure search. However, and encouragingly, our predicted lowest energy phase for the same stoichiometry calculates as only 48 meV/f.u. (1.1 kcal/mol) above the experimental structure (Figure ). We note that whereas our structure search has been extensive it cannot be exhaustive in part due to practical limitations in the sizes of unit cells described. Nonetheless, any missing ground state phase is likely to also feature rhodizonate ions.