Computational aspects of a local-MP2 treatment of electron correlation in periodic systems: SiC vs BeS

We present some aspects of the computational problems that are encountered with a local-correlation MP2 treatment of periodic systems, taking as an example SiC and BeS. The local functions which span the occupied and the virtual Hartree-Fock manifolds are described, focussing the attention on their locality features. The relative importance of different kinds of local excitations and their dependence on the prevailingly covalent or ionic character of the crystal is discussed. The usefulness of a multipolar approximation for the evaluation of the majority of 2-el integrals is demonstrated.