We present a study, employing high level ab initio methods, of electron localization-delocalization transitions along the dissociation path of the C-4(-) anion to C-2 and C-2(-). We find that at the equilibrium geometry, the symmetrical and nonsymmetrical configurations of the linear C-4(-) anion are almost isoenergetic. However, along a collinear dissociation path, the dipole moment drops abruptly to zero when the separation between the two middle carbon nuclei reaches about R=2.15 Angstrom. The dipole moment remains zero until about R=2.78 Angstrom, and then continuously increases as dissociation proceeds. This behavior is analogous to critical phenomena: The abrupt drop to zero of the dipole moment resembles a first-order phase transition, the later steady rise resembles a continuous phase transition. We show that a simple sub-Hamiltonian model, corresponding to the large-dimension limit for an electron in the field of four collinear carbon atoms, exhibits both kinds of phase transitions along the dissociation path. (C) 2004 American Institute of Physics.