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 methods, i.e., coupled-cluster (CCD, CCSD, LCCD, ACCD, QCISD, and LCCSD) and many-body perturbation theories [MBPT(2) and MBPT(3)], are formulated and implemented for infinitely extended one-dimensional lattices (polymers), by taking account of the periodic boundary conditions. We present the results of initial benchmark calculations and also investigate the spatial spread of electron correlation by plotting the atomic-orbital-based t1- and t2-amplitudes with respect to a unit cell parameter n. The two-electron integrals and t2-amplitudes decay as n−1 and n−3, leading to the n−3 convergence of the lattice summations for correlation energies.