Exploring contractor renormalization: Perspectives and tests on the two-dimensional Heisenberg antiferromagnet

Contractor renormalization (CORE) is a numerical renormalization method for Hamiltonian systems that has found applications in particle and condensed matter physics. There have been few studies, however, on further understanding of what exactly it does and its convergence properties. The current work has two main objectives. First, we wish to investigate the convergence of the cluster expansion for a two-dimensional Heisenberg antiferromagnet. This is important because the linked cluster expansion used to evaluate this formula nonperturbatively is not controlled by a small parameter. Here we present a study of three different blocking schemes which reveals some surprises and, in particular, leads us to suggest a scheme for defining successive terms in the cluster expansion. Our second goal is to present some new perspectives on CORE in light of recent developments to make it accessible to more researchers, including those in quantum information science. We make some comparison to entanglement-based approaches and discuss how it may be possible to improve or generalize the method.