Electronic correlation in cyclic polyenes. Behavior of approximate coupled-pair theories for large rings
Abstract
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 beta=2.5 eV, the ACP method breaks down for M >= 446, and the ACPQ method--for M>194. In the ACPQ method, for M>170, 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.
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