Highly accurate treatment of electron correlation in polymers: coupled-cluster and many-body perturbation theories

A series of accurate and size-extensive ab initio wavefunction-based method s, i.e., coupled-cluster (CCD, CCSD, LCCD, ACCD. QCISD, and LCCSD) and many -body perturbation theories [MBPT(2) and MBPT(3)], are formulated and imple mented for infinitely extended one-dimensional lattices (polymers), by taki ng account of the periodic boundary conditions. We present the results of i nitial benchmark calculations and also investigate the spatial spread of el ectron correlation by plotting the atomic-orbital-based t(1)- and t(2)-ampl itudes with respect to a unit cell parameter n. The two-electron integrals and t(2)-amplitudes decay as n(-1) and n(-3), leading to the n(-3) converge nce of the lattice summations for correlation energies. (C) 2001 Published by Elsevier Science B.V.