The P6/mmm phase of K3C6O6 (hereafter only K3C6O6) contrasts sharply with all other compounds identified on the K-CO phase diagram, and it even conflicts with chemical expectations. The rhodizonates are in this material formally radicals carrying three negative charges (-2.6 by our analysis, c.f., Table ). The anions are predicted to be perfectly planar, D6h symmetric, with equidistant C-C (1.46 Å) and C-O (1.27 Å) bonds. Instead of arranging themselves in an alternating fashion with respect to their K + counterions, these rhodizonates prefer to be close, stacking face-to-face directly and equidistantly on top of each other (Figure ). One-dimensional chains of radicals are expected to be unstable with respect to a spontaneous symmetry breaking. Such deformations, which occur to avoid degeneracy of electronic levels, are known as Peierls and Jahn-Teller distortions in physics and chemistry, respectively. The equidistant structure we predict in K3C6O6 does not appear to be an artefact of the level of theory. The inter-ring distances in K3C6O6 calculates as 3.38 Å at the PBE-D3(BJ) level of theory and closer still, 3.34 Å, in a HSE06-D3(BJ) calculation. This distance is considerably shorter than the sum of C or O van der Waals radii, and remarkable considering it is between triply charged (hence electrostatically repelling) anions. The effect of the vdW correction in our calculations is also only responsible for an inter-ring shortening of 0.09Å. The K + counterions are in this structure cubically coordinated to oxygen and are positioned in concentric circles around each stack of anions (Figure ). The same equidistant stacking is also recreated when HSE06-D3(BJ) optimization commences from a symmetry broken starting point. Our level of theory is clearly capable of converging to non-equidistant stackings of rhodizonates, as evident from the P1 phase of K3.5C6O6 (Figure ). In the latter phase, symmetry is broken into alternating pairs of rhodizonates, still organized in one-dimensional stacks (Figure ). We note that while K3C6O6 has been previously predicted by Lian et al., the phase has only been discussed in terms of its potential performance as an anode (K + storage) material, while its metallic nature and unusual structure have been overlooked. Lian et al have also predicted a different phase of K3.5C6O6 with Fmmm symmetry, which feature dimeric stacking of rhodizonates. We have identified the Fmmm phase in our structure search, but predict it to lie 0.1 eV/f.u above the P1 phase.