Electron Correlations in Molecules and Solids

A discussion is given of electronic correlations in molecules and solids. Emphasis is put on a recently developed local approach (LA) to the correlation problem within which one can treat both types of systems. A survey is given of various results which have been obtained up to now with that method. For example, with its help one can derive simple analytical expressions for various correlations in the ground state of hydrocarbon molecules. When applied to diamond one can study in detail the various correlation energy contributions to the cohesive energy or the energy gap. When a bond orbital approximation is made the calculations simplify so much that they can be done analytically. Finally we give a brief survey of the results which have been obtained for transition metals.