CI calculations on metallic and molecular hydrogen rings

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Abstract
To evaluate the importance of correlation effects in metallic and insulating polymers self-consistent field (SCF) and configuration interaction (CI) calculations were performed for equidistant (metallic) and alternant (insulating) hydrogen rings. Two main effects–increasing the number of electrons and changing the distance between the hydrogen atoms–were studied. Minimal basis set calculations were performed for 6-, 10-, 14-, 22-, and 30-membered rings, potential curves for the metallic and insulating H14 system have been computed with a double-zeta basis supplemented by polarization functions. The correlation energy of one-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 H2 molecules in the ring of about 5.5 a.u. The calculated value for the energy minimum suggests nonpairwise-additive contributions to the van der Waals interaction energy of seven H2 molecules. 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.