CORRELATION-ENERGY CALCULATIONS FOR INFINITE SYSTEMS

A survey is given of ab initio calculations for solids and polymers, which start from an SCF calculation and treat correlations by using local operators. The theory is formulated by employing a projection method. As regards applications, special attention is paid to elemental semiconductors and to polymers like polyethylene and polyacetylene. With the help of reduced Hamiltonians, insight can be gained into various correlation effects. This is demonstrated by a discussion of the problem of dimerization in polyacetylene. Finally, a formalism is described that allows for treating the effects of correlations on energy bands of solids. It is shown that qualitative new aspects of correlations enter when a quasiparticle in, e.g., a semiconductor is considered. The theory is applied to a calculation of the energy bands of silicon.