Calculations of HCCX (X = F, Cl, Br, I) were carried out with the development version of the Gaussian software package and employed the cc-pVTZ basis set. Ionization energies were calculated with coupled cluster methods (CCSD, EOMCCSD-IP) and electron propagator theory (EPT). CCSD was used to calculate optimized geometries of neutral HCCX and the X state of HCCX cations. EOMCCSD-IP energy calculations, recently added to Gaussian by one of the authors (MC), were used for the X and A states of the HCCX cations. Geometries were optimized with the Fletcher-Powell approach using numerical first derivatives and frequencies were calculated using double numerical differentiation of the energy. Spin-orbit coupling and vibrational anharmonicity corrections were not included in the present calculation.