We mapped the six-dimensional potential energy surface of the electronic ground state of HCCH(++)(X (3)Sigma(-)(g)) dication using the coupled cluster approach. This potential energy surface is incorporated later into perturbative and full variational treatments to solve the nuclear motions. We derived a set of spectroscopic data for HCCH(++), HCCD(++), and DCCD(++). Our calculations reveal the presence of anharmonic resonances even at low energies, which complicates their assignment by vibrational quantum numbers. In light of our theoretical vibrational spectra, we propose an assignment of the experimental vibrationally resolved valence double ionization spectra of HCCH, HCCD, and DCCD. These spectra are viewed to be mostly composed by a pure vibrational progression involving the CC stretching mode together with a second progression involving both the CC stretching and the bendings. (C) 2010 American Institute of Physics. [doi:10.1063/1.3431625]