Electromagnetic Observables and Open-Shell Nuclei from the In-Medium No-Core Shell Model
We present extensions and new developments of the in-medium no-core shell model (IM-NCSM), which is a novel ab initio many-method that merges the multi-reference in-medium similarity renormalization group (MR-IM-SRG) with the no-core shell model (NCSM)---two complementary and successful ab initi...
| Summary: | We present extensions and new developments of the in-medium no-core shell model (IM-NCSM),
which is a novel ab initio many-method that merges the
multi-reference in-medium similarity renormalization group (MR-IM-SRG) with the no-core shell model (NCSM)---two
complementary and successful ab initio many-body methods.
Within the IM-NCSM framework, the MR-IM-SRG employs a correlated NCSM reference state
and unitarily transforms observables such that the reference state is decoupled.
Consequently, the model-space convergence of a subsequent NCSM calculation
is substantially accelerated---demonstrating the power of the IM-SRG decoupling scheme---and
the IM-NCSM can treat nuclei that are out of reach for traditional NCSM calculations.
In earlier applications we already employed the IM-NCSM for addressing scalar observables w.r.t.
ground and excited states in even open-shell nuclei, however,
this initial formulation of the IM-NCSM had several restrictions that we eliminate in this work.
Due to the spherical formulation of the IM-SRG equations---which is mandatory from a
computational point of view---the
total angular momentum of the reference state is required to vanish and, thus,
the IM-NCSM was restricted to the treatment of even nuclei.
The particle-attached/particle-removed extension overcomes this restriction and makes odd nuclei
accessible.
Furthermore, the spherical formulation of the IM-SRG equations did
not account for non-scalar operators and, therefore, the
consistent transformation of electromagnetic observables was not possible.
By deriving and implementing the IM-SRG equations corresponding to non-scalar observables,
we open up the possibility to calculate electromagnetic observables from the IM-NCSM.
These observables are sensitive to different aspects of the wave functions and, therefore,
ideal for validating theoretical models and new opportunities
for fruitful collaborations with experimentalists are opened up.
For the transformation of observables we implemented a Magnus-type transformation, which
determines the generator for the IM-SRG transformation and
greatly reduces the computational effort.
Considering numerical applications,
we employ the IM-NCSM for the calculation of ground-state energies, excitation energies,
radii, magnetic dipole moments, electric quadrupole moments, B(M1) transitions, and
B(E2) transitions, where we study various medium-mass nuclei up to calcium isotopes.
These calculations are already converged at very small
model-space sizes---showing the great advantage of the IM-NCSM---and the results are compatible
with large-scale NCSM calculations.
These applications demonstrate that the IM-NCSM is now capable of addressing the full range of nuclear structure
observables---including spectroscopic and electromagnetic observables---in fully open-shell nuclei. |
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