Journal
PHYSICAL REVIEW C
Volume 95, Issue 4, Pages -Publisher
AMER PHYSICAL SOC
DOI: 10.1103/PhysRevC.95.044304
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Funding
- National Science Foundation [PHY-1404159]
- NUCLEI SciDac Collaboration under DOE [DE-SC000851]
- Office of Nuclear Physics, U.S. Department of Energy [DE-SC0008499]
- Office of Science of the Department of Energy [DE-AC05-00OR22725]
- Direct For Mathematical & Physical Scien
- Division Of Physics [1565546] Funding Source: National Science Foundation
- Direct For Mathematical & Physical Scien
- Division Of Physics [1404159] Funding Source: National Science Foundation
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We present two new methods for performing ab initio calculations of excited states for closed-shell systems within the in-medium similarity renormalization group (IMSRG) framework. Both are based on combining the IMSRG with simple many-body methods commonly used to target excited states, such as the Tamm-Dancoff approximation (TDA) and equations-of-motion (EOM) techniques. In the first approach, a two-step sequential IMSRG transformation is used to drive the Hamiltonian to a form where a simple TDA calculation (i.e., diagonalization in the space of 1p1h excitations) becomes exact for a subset of eigenvalues. In the second approach, EOM techniques are applied to the IMSRG ground-state-decoupled Hamiltonian to access excited states. We perform proof-of-principle calculations for parabolic quantum dots in two dimensions and the closed-shell nuclei O-16 and O-22. We find that the TDA-IMSRG approach gives better accuracy than the EOM-IMSRG when calculations converge, but it is otherwise lacking the versatility and numerical stability of the latter. Our calculated spectra are in reasonable agreement with analogous EOM-coupled-cluster calculations. This work paves the way for more interesting applications of the EOM-IMSRG approach to calculations of consistently evolved observables such as electromagnetic strength functions and nuclear matrix elements, and extensions to nuclei within one or two nucleons of a closed shell by generalizing the EOM ladder operator to include particle-number nonconserving terms.
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