To evaluate the importance of correlation effects in metallic and insulating polymers selfhyphen;consistent field (SCF) and configuration interaction (CI) calculations were performed for equidistant (metallic) and alternant (insulating) hydrogen rings. Two main effectsndash;increasing the number of electrons and changing the distance between the hydrogen atomsndash;were studied. Minimal basis set calculations were performed for 6hyphen;, 10hyphen;, 14hyphen;, 22hyphen;, and 30hyphen;membered rings, potential curves for the metallic and insulating H14system have been computed with a doublehyphen;zeta basis supplemented by polarization functions. The correlation energy of onehyphen;dimensional metallic hydrogen is estimated to be about 0.84 eV/atom compared with 0.54 eV in H2. For molecular hydrogen a van der Waals minimum was found for separation of two H2molecules in the ring of about 5.5 a.u. The calculated value for the energy minimum suggests nonpairwisehyphen;additive contributions to the van der Waals interaction energy of seven H2molecules. Finally, the pressure to induce a transition from the molecular to the metallic phase was calculated to be about 1.8 Mbar both on the SCF and CI level.
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