Fast coupled-cluster singles and doubles for extended systems: Application to the anharmonic vibrational frequencies of polyethylene in the Γ approximation

The mod-n scheme is introduced to the coupled-cluster singles and doubles (CCSD) and third-order Moller-Plesset perturbation (MP3) methods for extended systems of one-dimensional periodicity. By downsampling uniformly the wave vectors in Brillouin-zone integrations, this scheme accelerates these accurate but expensive correlation-energy calculations by two to three orders of magnitude while incurring negligible errors in their total and relative energies. To maintain this accuracy, the number of the nearest-neighbor unit cells included in the lattice sums must also be reduced by the same downsampling rate (n). The mod-n CCSD and MP3 methods are applied to the potential-energy surface of polyethylene in anharmonic frequency calculations of its infrared- and Raman-active vibrations. The calculated frequencies are found to be within 46 cm(-1) (CCSD) and 78 cm(-1) (MP3) of the observed.