  The Similarity Renormalization Group (SRG) is investigated as a powerful yet
practical method to modify nuclear potentials so as to reduce computational
requirements for calculations of observables. The key feature of SRG
transformations that leads to computational benefits is the decoupling of
low-energy nuclear physics from high-energy details of the inter-nucleon
interaction. We examine decoupling quantitatively for two-body observables and
few-body binding energies. The universal nature of this decoupling is
illustrated and errors from suppressing high-momentum modes above the
decoupling scale are shown to be perturbatively small. To explore the SRG
evolution of many-body forces, we use as a laboratory a one-dimensional system
of bosons with short-range repulsion and mid-range attraction, which emulates
realistic nuclear forces. The free-space SRG is implemented for few-body
systems in a symmetrized harmonic oscillator basis using a recursive
construction analogous to no-core shell model implementations. Building on
one-dimensional results we performed the first practical evolution of
three-dimensional many-body forces within the No-Core Shell Model basis.
Results for the 3H binding energy are consistent with previous calculations
involving momentum-space evolution of only two-body forces, and validate
expectations from calculations in the one-dimensional oscillator basis. When
applied to 4He calculations, the two- and three-body oscillator matrix elements
yield rapid convergence of the ground-state energy with a small net
contribution of the induced four-body force. The radius of light nuclei is also
explored in the three-dimensional basis.

Nuclear Theory