Modern correlation theories for extended, periodic systems

Recent progress on correlated methods for extended, periodic systems is bri efly summarized. Special difficulties compared to finite systems such as co nvergence with infinite summations over lattices and integrations over reci procal vectors are addressed. We also consider the comparative advantages o f Bloch orbitals and Wannier orbitals. The applications of MBPT(2) to polym ers for determining their equilibrium structures, vibrational frequences, X PS and UPS spectra, and band gaps are examined. It is shown that MBPT(2) co rrections have significant contributions to all of the above quantities and make significant improvements to the HF results, especially for the XPS an d UPS spectra.