We report the isomerization energies of cumulene and poly-yne oligomeric sequences calculated using several different theoretical methods in an attempt to evaluate both the performance of these methods and their potential application to similar systems. We find that the recently developed KMLYP density functional theory method reproduces the CCSD(T) benchmark relative energies better than other commonly used quantum chemical methods. Furthermore, the KMLYP relative energies scale significantly better with molecule length with an average error of 0.6 kcal/mol per additional C-2 monomer. The B3LYP, B3PW91, mPW1PW91, and BXLYP methods scale with errors of 2.3, 2.4, 2.0, and 2.1 kcal/mol per additional C-2 monomer, respectively, while the MP2, MP4(SDQ), and CCSD methods scale with errors of 2.6, 1.4, and 1.4 kcal/mol, respectively. Consequently, these methods have large errors for chain lengths above C-5. The Hartree-Fock (HF) method is surprisingly successful in calculating the enthalpy difference between shortest eumulene/poly-yne isomers, allene and propyne. This appears to be the result of a fortuitous equivalence of correlation energies for these two molecules as HF adds an additional error of 5.1 kcal/mol per additional C-2 unit. We point out how this equivalence makes the allene/propyne system useful as a testing ground for the ability of quantum chemical methods to capture correlation energy.