The vibrational frequencies for neutral HCCX are listed in Table . The calculated harmonic frequencies differ from the observed frequencies by about 5%, primarily because of vibrational anharmonicity. As X is changed from F to Cl, Br and I, the calculated frequencies reproduce the trends in the observed frequencies for each of the normal modes. The good agreement between the calculations and experiment suggests that the CCSD/cc-pVTZ method should be a satisfactory level of theory for calculating the vibrational frequencies for the cations. HCCCl, HCCBr, The calculated and observed vibrational frequencies for the ground state and first excited state of HCCX + (X and A states, respectively) are listed in Tables and. Experimentally, the HCCCl, HCCBr and HCCI cations are generated by electron impact in a supersonically cooled jet and the vibrational frequencies are obtained from the emission spectra. No experimental data is available for HCCF, presumably because the A state of HCCF + is not emissive. Spin-orbit coupling, vibrational resonances and Renner-Teller effects make the analysis of these spectra very challenging. For the X state of HCCX + , vibrational frequencies have been calculated with both the CCSD and EOMCC-IP approaches. The agreement between the two sets of calculated frequencies is quite good, differences ranging from -3 cm -1 to 32 cm -1 with a mean absolute deviation of 13 cm -1 . The CC triple bond stretching frequencies calculated with EOMCCSD-IP are systematically 15 -32 cm -1 higher than with CCSD. This is because the CC bond lengths are 0.007 Å shorter with EOMCCSD-IP, possibly reflecting the greater difficulty in treating the electron correlation in the triple bond. The CX stretching frequencies calculated with both methods agree very well with the observed frequencies. linear; for trans bent HCCF + , frequencies (cm -1 ):  (A') = 3377,  (A') = 2061,  (A') = 746,  (A') = 719,  (A') = 233,  (A") = 720; b HCCCl, HCCBr, The frequencies for the A states of HCCX + can be computed with EOMCCSD-IP but not with conventional CCSD. The availability of experimental vibration frequencies for the A states is more limited than for the X state. The differences between calculated and observed frequencies for CC and CX stretch are a bit larger for HCCI + than for HCCBr + and HCCCl + . The largest change in frequency and in geometry on going from the X state to the A state is in the C-X stretch. The calculated and experimental CX stretching frequencies for the A state are lower than for the X state, reflecting the longer CX bond in the A state. This effect increases as the halogen is changed from I to Br to Cl. Differences between the X and A states in the C-I bond length and C-I stretching frequency are key factors that determine the decay time for coherence and charge migration in HCCI. The calculated and experimental frequencies are compared in Figure . Overall, the agreement between the calculated and observed frequencies is very good. A linear least-squares fit with zero intercept yields calc = 1.0505 exp (R 2 = 0.9993, 61 data points). The calculated frequencies are about 5% higher than observed primarily because anharmonicity corrections are not included in the calculations. Only a few vibrations deviate significantly from the linear correlation. These include the CC triple bond stretching mode in the X and A states of HCCI + and the CCH bending mode in the X state of HCCI + . Potentially, this could be due to the larger spin-orbit coupling in HCCX + than in the other haloacetylene cations. The experimental splittings between the J = 3/2 and J = 1/2 spin orbit states of HCCX (X = Cl, Br and I) are 0.02, 0.13 and 0.40 eV, resp., for the X states and 0.05, 0.20 and 0.25 eV, resp., for the A states. The CCH and CCX bending region, 200 -700 cm -1 , shows a little more deviation than other regions. These bending modes are more difficult to measure experimentally and are subject to Renner-Teller perturbations. Figure . Comparison of the calculated harmonic frequencies with the observed frequencies for neutral HCCX (X = F, Cl, Br and I) and X and A states of the HCCX cations (red circles: CCSD/cc-pVTZ.; blue triangles EOMCCSD-IP/cc-pVTZ; calc = 1.0505 exp, R 2 = 0.9993)