Dynamics and applicability of the similarity renormalization group

The similarity renormalization group (SRG) concept (or flow equations methodology) is studied with a view toward the renormalization of nucleon-nucleon interactions for ab initio shell-model calculations. For a general flow, we give quantitative measures, in the framework of spectral distribution theory, for the strength of the SRG-induced higher order (many-body) terms of an evolved interaction. Specifically, we show that there is a hierarchy among the terms, with those of the lowest particle rank being the most important. This feature is crucial for maintaining the unitarity of SRG transformations and key to the method's applicability.