The Variational Cluster Approximation for Hubbard Models: Practical Implementation

The Variational cluster approximation (VCA) is an approximate method for finding properties the Hubbard model, the basic model used in the study of strongly correlated electron systems. Such systems include, for instance, high-temperature and organic superconductors. The method involves solving exactly the Hubbard model on a small cluster of lattice sites, after adding fields that represent the effect of the cluster´s environment. We present an outline of the method and describe its implementation. In particular, we show how to efficiently carry frequency- wavevector integrals that are central to the method.