The equilibrium molecular structure of the octatetranyl anion, CsH-, which has been recently detected in two astronomical environments, is investigated with the aid of both ab initio post- Hartree-Fock and density functional theory (DFT) calculations. The model chemistry adopted in this study was selected after a series of benchmark calculations performed on molecular acetylene for which accurate gas-phase structural data are available. Geometry optimizations performed at the CCSD/6-31 I +G(2d,p), QCISD/6-31 I +G(2d,p), and MP4(SDQ)/6-31 I +G(2d,p) levels of theory yield for C8H- an interesting polyyne-type structure that defies the chemical formula displaying a simple alternation of triple and single carbon-carbon bonds, [:C=-C-C=-C-C=-C-C=-CH]'-. In the optimized geometry Of C8H-, as one proceeds from the naked carbon atom on one side of the chain to the CH unit on the opposite side of the chain, the short (formally triple) carbon-carbon bonds decrease in length from 1.255 to 1.213 A whereas the long (formally single) carbon-carbon bonds increase (albeit only slightly) in length from 1.362 to 1.378 A (CCSD results). In striking contrast, both MP2 and DFT (B3LYP and PBEO) calculations fail in reproducing the pattern of the carbon-carbon bond lengths obtained with the CCSD, QCISD, and MP4 methods. The structures of three shorter n-even chains, C,H- (n = 2, 4, and 6), along with those of four n-odd compounds (n = 3, 5, 7, and 9) are also investigated at the CCSD/6-31 I +G(2d,p) level of theory.