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Abstract
Four compounds of simple molecular systems have already been observed at high pressure: He(N2)1 1, Ne(He)2, Ar(H2)2 and (O2)3 (H2)4 They have been characterized by a combination of visual, Raman and energy dispersive measurements. Yet, the direct x-ray determination of their space-groups remains an experimental challenge. Gibbs free energy calculations demonstrate that efficient packing more than close packing is the reason of their stability. Finally, those compounds offer interesting issues for the physical chemistry at high pressure and astrophysical implications for outer planets.