Figure 1: Ortho and para water inside C₆₀.

(a) Molecular structure of the endofullerene H₂O@C₆₀ and (b) the lowest rotational energy levels, in temperature units^1,3. The small splitting of the ortho-H₂O ground state is ignored for simplicity. (c) Blue line: fraction of ortho-water Φ _eq as a function of temperature in thermal equilibrium.