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Density functional theory is used to carry out a systematic study of zero-temperature structural and energy properties of endohedrally confined hydrogen clusters as a function of pressure and the cluster size. At low pressures, the most stable structural forms of (H(2))(n) possess rotational symmetry that changes from C(4) through C(5) to C(6) as the cluster grows in size from n=8 through n=12 to n=15. The equilibrium configurational energy of the clusters increases with an increase of the pressure. The rate of this increase, however, as gauged on the per atom basis is different for different clusters sizes. As a consequence, the size dependencies of the configurational energies per atom at different fixed values of pressure are nonmonotonic functions. At high pressures, the molecular (H(2))(n) clusters gradually become atomic or dominantly atomic. The pressure-induced changes in the HOMO-LUMO gap of the clusters indicate a finite-size analog of the pressure-driven metallization of the bulk hydrogen. The ionization potentials of the clusters decrease with the increase of pressure on them.

Embedded cluster calculations at the MP2 electron correlation level of pure and Zn-and Ni-doped YBa 2 Cu 3 O 7 (Y123) ceramics reveal great changes of the local charge distribution and crystal bonding in the vicinity of impurities. In the Zn-doped ceramics the obtained changes are stronger and more extended than in the Ni-doped ceramics. This can be one of explanations of the stronger depression of T c due to Zn. The calculated increase of positive charge for both impurities, comparing with substituted Cu, indicates that the impurity atoms lose more negative charge than Cu. As a result, all nearest CuO 2 units around impurity lose their holes: completely in the Zn case and partly in the Ni case. This leads to the breaking of the hole pairs in the vicinity of impurity and correlated with an increase of the coherence length in Zn-and Ni-doped Y123 crystals measured by Timomoto et al. ͓Phys. Rev. B 60, 114 ͑1999͔͒. Both Zn and Ni impurities, the latter to a less extent, act as effective pair breakers due to the elimination of holes on adjacent CuO 2 groups.

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