The ionization potentials (IP) are listed in Tables 4. The experimental adiabatic IP's for HCCCl, HCCBr and HCCI cations are for the lower energy J = 3/2 spin-orbit states. The calculated vertical IP's are obtained from the electronic energy difference at the geometry of the neutral molecule. Electron propagator theory (EPT) provides an efficient way to compute vertical IP's that is comparable to 4 th order many body perturbation theory and yields values within about 0.2 eV of the experimental adiabatic IP's. The CCSD and EOMCCSD-IP vertical IP's are also in very good agreement with the experiment. The CCSD values are lower than EOMCCSD-IP because the CCSD wavefunction is somewhat more flexible. The adiabatic IP's are calculated as the enthalpy difference at 0 K between the ion and the neutral, each at their respective minimum energy geometries. Relaxation of the ion structures to their equilibrium geometries lowers the energies by approximately 0.2 eV. The CCSD and EOMCCSD-IP calculated adiabatic IP's are within about 0.2 eV of the experimental adiabatic IP's. As noted above, the A state of HCCF is a special case. Relaxation of the linear geometry lowers the energy by 0.90 eV. Further relaxation to the bent structure (see footnote in Table ) lowers the energy by an additional 0.032 eV and yields an adiabatic IP of 17.174 eV. The frequency for charge migration in HCCX cations depends on the energy difference between the X and A states. Accurate experimental energy differences between X and A cation states in their ground vibrational state have been obtained from analysis of the A to X emission spectra. The EOMCCSD-IP calculations are in very good agreement, with differences of -0.016 to 0.087 eV.