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Molecular oxygen has been detected in the coma of comet 67P/Churyumov-Gerasimenko with abundances in the 1%-10% range by the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis-Double Focusing Mass Spectrometer instrument on board the Rosetta spacecraft. Here we find that the radiolysis of icy grains in lowdensity environments such as the presolar cloud may induce the production of large amounts of molecular oxygen. We also show that molecular oxygen can be efficiently trapped in clathrates formed in the protosolar nebula (PSN), and that its incorporation as crystalline ice is highly implausible, because this would imply much larger abundances of Ar and N 2 than those observed in the coma. Assuming that radiolysis has been the only O 2 production mechanism at work, we conclude that the formation of comet 67P/Churyumov-Gerasimenko is possible in a dense and early PSN in the framework of two extreme scenarios: (1) agglomeration from pristine amorphous icy grains/ particles formed in ISM and (2) agglomeration from clathrates that formed during the disk's cooling. The former scenario is found consistent with the strong correlation between O 2 and H 2 O observed in comet 67P/ChuryumovGerasimenko's coma while the latter scenario requires that clathrates formed from ISM icy grains that crystallized when entering the PSN.

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Slab and cluster model spin-polarized calculations have been carried out to study various properties of isolated first-row transition metal atoms adsorbed on the anionic sites of the regular MgO͑100͒ surface. The calculated adsorption energies follow the trend of the metal cohesive energies, indicating that the changes in the metal-support and metal-metal interactions along the series are dominated by atomic properties. In all cases, except for Ni at the generalized gradient approximation level, the number of unpaired electron is maintained as in the isolated metal atom. The energy required to change the atomic state from high to low spin has been computed using the PW91 and B3LYP density-functional-theory-based methods. PW91 fails to predict the proper ground state of V and Ni, but the results for the isolated and adsorbed atom are consistent within the method. B3LYP properly predicts the ground state of all first-row transition atom the high-to low-spin transition considered is comparable to experiment. In all cases, the interaction with the surface results in a reduced high-to low-spin transition energy.

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The present study is devoted to atomic adsorption of Ni on MgO (100) surface and focuses on the magnetic moment of adsorbed atomic Ni in the initial phase of metal deposition. Both cluster and periodic slab models lead to the same description, i.e., they show an almost degeneracy of the magnetic and nonmagnetic states. However, the total energy related properties are strongly dependent on the computational method chosen. At variance with the large difference for the calculated heats of adsorption, the optimized values for the Ni–O distances are remarkably close: 1.8–1.9 Å for the singlet state and larger than 2 Å for the triplet. The paper also presents a detailed analysis of the problems connected to the definition of a proper reference atomic energy in density functional theory approaches.