Orbital Removal via In-Medium Similarity Renormalization Group

The in-medium similarity renormalization group (IMSRG) is a powerful eigenstate solver in low-energy nuclear physics. The IMSRG utilizes normal-ordering to capture beyond-mean-field effects and three-body forces, and then uses a sequence of continuous unitary transformations to find the desired eigenstate. The IMSRG has found great success in being applied to medium-mass nuclei, and more recently, heavy nuclei and infinite nuclear matter. The polynomial scaling of IMSRG with basis size makes it computationally comparable to other many-body methods such as coupled-cluster and Green’s function approaches. However, the combinatorics of the nuclear many-body problem are still prohibitive for running large calculations.

In this presentation I will discuss a recent algorithmic improvement for the IMSRG, which we have dubbed as orbital removal. I will discuss how this procedure for removing orbitals/shells using IMSRG transformations to reduce the size of the many-body configuration space can provide computational savings at the cost of small, controlled losses in accuracy. I will show preliminary results using the Richardson pairing model that demonstrate the computational speedup offered by this procedure. This procedure shows promise in accelerating large-scale calculations of infinite nuclear matter, a task that has thus far been hampered by computational power.