Perturbation Expansions for Quantum Many Body Systems.

A systematic method for developing high-order, zero-temperature perturbation expansions for quantum many-body systems is presented. The models discussed explicitly are spin models with a variety of interactions, in one and two dimensions. The wide applicability of the method is illustrated by expansions around Hamiltonians with ordered and disordered ground states, namely, Ising and dimerized models. Computer implementation of this method is discussed in great detail. Some previously unpublished series are tabulated.