Coupled-cluster singles and doubles for extended systems

The formalisms and computer implementation of size-extensive, highly accurate models of electronic structure of polymers that were CCSD, LCCSD, QCISD, CCD, LCCD, ACCD, MBPT(2) and MBPT(3) were discussed. The convergence of the correlation energies reflected the electronic structure such as the degree of π-electron conjugation. It was found that the t ₂ and t ₁ amplitudes of nonconjugated polyethylene decay noticeably faster than those of conjugated polyacetylene and polyyne. Analysis shows tha the rate of convergence of t ₁ amplitudes was particularly slow for conjugated polymers.