Many-body perturbation theory with localized orbitals - Kapuy's approach

In this contribution the application of localized molecular orbitals for th e separation of local and long-range correlation effects in extended system s is studied in the framework of the many-body perturbation theory. We firs t summarize the basic ideas developed by Professor Kapuy for extending diag rammatic methods based on localized one-electron states in correlation ener gy calculations. After describing some possible ways for characterizing the extension and separation of localized MOs we give a flexible procedure for the truncation of long-range correlation effects with the remarkable prope rty that the range of the Coulomb interaction is still kept infinite. Analy zing numerical results the convergence of localization corrections is discu ssed and the separation of local correlation terms show that only the immed iate neighborhood of a localized MO plays a considerable role in excitation processes.