Out of the 16,319 materials in MP that belong to the 4-element chemical space, there are 12,607 materials that lack at least one 3-element subset chemical space within the database. Figure displays how many 3-element subset chemical spaces are lacking for the entire 4element chemical spaces in MP. The fact that 82 of the 4-element chemical spaces lack all the 3-element subset chemical spaces is interesting. An example of those 82 is {Cu, Eu, F, Se}, for which the MP has only one entry, the layered structure EuCuSeF (mp-21356) which has a reported E h value of 0 eV. None of the possible 3-element chemical spaces, namely {Cu, Eu, F}, {Cu, Fe, Se}, {Cu, Eu, Se}, and {Eu, F, Se}, have any corresponding materials in the MP database. The database also reports that the material is "not predicted to decompose", but this is because of the lack of the 3-element subset chemical spaces we described, not necessarily because the material is thermodynamically stable. That is, it is possible that E h goes above 0 eV once 3-element chemical spaces, such as {Cu, Se, F} are introduced. Vertical: There could be more stable phases for a given chemical space which we are not aware of yet There is, in principle, an unlimited number of possible spatial configurations that can be used to arrange the atoms within any chemical space C. Nature can verify which of these configurations, or phases, should be stable. However, the first principles method used for optimising the structures can reveal lower energy phases and stoichiometriesthe natural presence of a phase does not imply that its E h is zero. For example, while it is established that graphite (mp-169) with AB stacking is the most stable phase of carbon, the MP database enumerates two phases (mp-2516584 and mp-569304) that have different stacking configurations.