HPEs represent only one (relatively small) class of electrides. Perhaps the most famous electrides are the organic ones (e.g. Cs + 18C6•e -(cesium 18-crown-6 electride) and K(cryptand[2.2.2]) ), whose studies were pioneered by James Dye. Related compounds such as alkalides (where alkali metal anions take the place of the interstitial electrons ), solvated electrons (e.g. alkali metals dissolved in ammonia), and expanded metals (compounds made from "superatomic" building blocks such as Li(NH 3 ) 4 ) have also been studied intently, though mostly at 1 atm. Inorganic systems including [Ca 24 Al 28 O 68 ] 4+ 4e 4-71 and 2-dimensional Ca 2 N 72 are often described as electrides, though these descriptions may not be accurate. In fact, a rigorous set of rules that can be used to unambiguously recognize an electride based upon the results of quantum mechanical calculations has not been followed. As a result, sometimes claims of "electrideness" are unsubstantiated.