ELECTRONIC-STRUCTURES OF LINEAR-C4, LINEAR-C6, LINEAR-C8, AND LINEAR-C10 CARBON CLUSTERS AND A SYMMETRY-BREAKING PHENOMENON

The electronic structures of linear C4, C6, C8, and C10 carbon clusters are studied using the self-consistent-field (SCF) and single and double configuration interaction (CISD) methods in conjunction with a double-zeta plus polarization basis set. The relative energies of various electronic states and that of two distinct linear geometries, cumulenic and acetylenic, are considered. It is found that the energy difference between the lowest triplet and singlet states of the cumulenic structure and that between the cumulenic and acetylenic structures decrease monotonically as the carbon chain lengthens. For C8 and C10 these energy differences are very small. A symmetry breaking phenomenon is observed in the 1Ag component (in D2h) of the 1Δg singlet state. Its symmetry-adapted, D∞h symmetric SCF wave function is unstable and the symmetry-broken, D2h symmetric solution has lower energy. © 1990.