9 research outputs found
Open-Shell Nuclei and Excited States from Multi-Reference Normal-Ordered Hamiltonians
We discuss the approximate inclusion of three-nucleon interactions into ab
initio nuclear structure calculations using a multi-reference formulation of
normal ordering and Wick's theorem. Following the successful application of
single-reference normal ordering for the study of ground states of closed-shell
nuclei, e.g., in coupled-cluster theory, multi-reference normal ordering opens
a path to open-shell nuclei and excited states. Based on different
multi-determinantal reference states we benchmark the truncation of the
normal-ordered Hamiltonian at the two-body level in no-core shell-model
calculations for p-shell nuclei, including 6-Li, 12-C, and 10-B. We find that
this multi-reference normal-ordered two-body approximation is able to capture
the effects of the 3N interaction with sufficient accuracy, both, for
ground-state and excitation energies, at the computational cost of a two-body
Hamiltonian. It is robust with respect to the choice of reference states and
has a multitude of applications in ab initio nuclear structure calculations of
open-shell nuclei and their excitations as well as in nuclear reaction studies.Comment: 6 pages, 4 figures, v2: update to published versio
Ab Initio Description of Open-Shell Nuclei: Merging No-Core Shell Model and In-Medium Similarity Renormalization Group
We merge two successful ab initio nuclear-structure methods, the no-core
shell model (NCSM) and the multi-reference in-medium similarity renormalization
group (IM-SRG) to define a new many-body approach for the comprehensive
description of ground and excited states of closed and open-shell nuclei.
Building on the key advantages of the two methods---the decoupling of
excitations at the many-body level in the IM-SRG and the access to arbitrary
nuclei, eigenstates, and observables in the NCSM---their combination enables
fully converged no-core calculations for an unprecedented range of nuclei and
observables at moderate computational cost. We present applications in the
carbon and oxygen isotopic chains, where conventional NCSM calculations are
still feasible and provide an important benchmark. The efficiency and rapid
convergence of the new approach make it ideally suited for ab initio studies of
the complete spectroscopy of nuclei up into the medium-mass regime.Comment: 5 pages, 4 figures, v2: update to published versio
Open-shell nuclei from No-Core Shell Model with perturbative improvement
We introduce a hybrid many-body approach that combines the flexibility of the No-Core Shell Model (NCSM) with the efficiency of Multi-Configurational Perturbation Theory (MCPT) to compute groundand excited-state energies in arbitrary open-shell nuclei in large model spaces. The NCSM in small model spaces is used to define a multi-determinantal reference state that contains the most important
multi-particle multi-hole correlations and a subsequent second-order MCPT correction is used to capture additional correlation effects from a large model space. We apply this new ab initio approach for the calculation of ground-state and excitation energies of even and odd-mass carbon, oxygen, and fluorine isotopes and compare to large-scale NCSM calculations that are computationally much more expensive
In-Medium No-Core Shell Model for Ab Initio Nuclear Structure Calculations
In this work, we merge two successful ab initio nuclear-structure methods, the no-core shell model (NCSM) and the multi-reference in-medium similarity renormalization group (IM-SRG), to define a novel many-body approach for the comprehensive description of ground and excited states of closed- and open-shell medium-mass nuclei.
Building on the key advantages of the two methods — the decoupling of excitations at the many-body level in the IM-SRG, and the exact diagonalization in the NCSM applicable up to medium-light nuclei — their combination enables fully converged no-core calculations for an unprecedented range of nuclei and observables at moderate computational cost. The efficiency and rapid model-space convergence of the new approach make it ideally suited for ab initio studies of ground and low-lying excited states of nuclei up to the medium-mass regime.
Interactions constructed within the framework of chiral effective field theory provide an excellent opportunity to describe properties of nuclei from first principles, i.e., rooted in quantum chromodynamics, they overcome the lack of predictive power of phenomenological potentials. The hard core of these interactions causes strong short-range correlations, which we soften by using the similarity-renormalization-group transformation that accelerates the model-space convergence of many-body calculations. Three-nucleon effects, which are mandatory for the correct description of bulk properties of nuclei, are included in our calculations by using the normal-ordered two-body approximation, which has been shown to be sufficient to capture the main effects of the three-nucleon interaction.
Using these interactions, we analyze energies of ground and excited states in the carbon and oxygen isotopic chains, where conventional NCSM calculations are still feasible and provide an important benchmark. Furthermore, we study the Hoyle state in C-12 — a three-alpha cluster state that cannot be converged in standard NCSM calculations. Moreover, we explore island-of-inversion physics in magnesium isotopes, where the shell-model magic numbers vanish and new ones appear.
Due to our implementation of the IM-NCSM method, we are restricted to nuclei with even mass numbers. We propose and benchmark a simple and straightforward idea for the extension to odd nuclei within the framework of IM-NCSM using a particle-attached or particle-removed scheme
In-Medium No-Core Shell Model for Ab Initio Nuclear Structure Calculations
In this work, we merge two successful ab initio nuclear-structure methods, the no-core shell model (NCSM) and the multi-reference in-medium similarity renormalization group (IM-SRG), to define a novel many-body approach for the comprehensive description of ground and excited states of closed- and open-shell medium-mass nuclei.
Building on the key advantages of the two methods — the decoupling of excitations at the many-body level in the IM-SRG, and the exact diagonalization in the NCSM applicable up to medium-light nuclei — their combination enables fully converged no-core calculations for an unprecedented range of nuclei and observables at moderate computational cost. The efficiency and rapid model-space convergence of the new approach make it ideally suited for ab initio studies of ground and low-lying excited states of nuclei up to the medium-mass regime.
Interactions constructed within the framework of chiral effective field theory provide an excellent opportunity to describe properties of nuclei from first principles, i.e., rooted in quantum chromodynamics, they overcome the lack of predictive power of phenomenological potentials. The hard core of these interactions causes strong short-range correlations, which we soften by using the similarity-renormalization-group transformation that accelerates the model-space convergence of many-body calculations. Three-nucleon effects, which are mandatory for the correct description of bulk properties of nuclei, are included in our calculations by using the normal-ordered two-body approximation, which has been shown to be sufficient to capture the main effects of the three-nucleon interaction.
Using these interactions, we analyze energies of ground and excited states in the carbon and oxygen isotopic chains, where conventional NCSM calculations are still feasible and provide an important benchmark. Furthermore, we study the Hoyle state in C-12 — a three-alpha cluster state that cannot be converged in standard NCSM calculations. Moreover, we explore island-of-inversion physics in magnesium isotopes, where the shell-model magic numbers vanish and new ones appear.
Due to our implementation of the IM-NCSM method, we are restricted to nuclei with even mass numbers. We propose and benchmark a simple and straightforward idea for the extension to odd nuclei within the framework of IM-NCSM using a particle-attached or particle-removed scheme
Open-shell nuclei from No-Core Shell Model with perturbative improvement
We introduce a hybrid many-body approach that combines the flexibility of the No-Core Shell Model (NCSM) with the efficiency of Multi-Configurational Perturbation Theory (MCPT) to compute ground- and excited-state energies in arbitrary open-shell nuclei in large model spaces. The NCSM in small model spaces is used to define a multi-determinantal reference state that contains the most important multi-particle multi-hole correlations and a subsequent second-order MCPT correction is used to capture additional correlation effects from a large model space. We apply this new ab initio approach for the calculation of ground-state and excitation energies of even and odd-mass carbon, oxygen, and fluorine isotopes and compare to large-scale NCSM calculations that are computationally much more expensive. Keywords: Perturbation theory, Ab initio, Many-body theor