Electronic correlation in cyclic polyenes. Behavior of approximate coupled-pair theories for large rings

We investigate the cyclic polyenes (annulenes) CMHM, described by the Pariser–Parr–Pople (PPP) model, by means of the approximate coupled-pair theories (ACP, ACPQ). For the systems with the spectroscopic value of the PPP resonance integral β=2.5 eV, the ACP method breaks down for M⩾446 and the ACPQ method—for M⩾198. In the ACPQ method, for M⩾174, two close lying solutions have been observed that become quasi-degenerate for M⩾198. The results indicate that the ACP and ACPQ methods cannot be applied to the one-dimensional metallic case.