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. © 1982 American Institute of Physics.