Coupled-cluster and configuration-interaction calculations for odd-A heavy nuclei

Area of Science:

• Nuclear Physics
• Quantum Chemistry

Background:

• Accurate theoretical descriptions of atomic nuclei are crucial for understanding nuclear structure and reactions.
• Coupled-cluster (CC) and configuration-interaction (CI) are key computational methods in nuclear physics.
• Investigating medium-mass nuclei like Nickel isotopes (55Ni, 57Ni) presents challenges due to strong electron correlation effects.

Purpose of the Study:

• To compare the accuracy and efficiency of various equation-of-motion coupled-cluster (EOM-CC) approximations against configuration-interaction (CI) methods.
• To evaluate these methods for describing the nuclear structure of 55Ni and 57Ni within the pf-shell model space.
• To assess the impact of correlation effects, influenced by orbital energy gaps, on the performance of CC and CI calculations.

Main Methods:

• Utilized the pf-shell basis for nuclear structure calculations.
• Employed equation-of-motion coupled-cluster (EOM-CC) approximations with varying excitation levels (up to 2p-2h in the cluster operator and 3p-2h/3h-2p in the excitation operator).
• Compared EOM-CC results with configuration-interaction (CI) calculations truncated at 3p-3h and 4p-4h excitations.

Main Results:

• EOM-CC methods with up to 2p-2h cluster components and 3p-2h/3h-2p excitation components demonstrated superior accuracy compared to CI methods with up to 3p-3h excitations.
• The accuracy of CC methods was found to be nearly as good as more computationally expensive CI approaches truncated at 4p-4h excitations.
• The performance of CC methods remained robust irrespective of the energy gap between the f7/2 and other valence orbits (f5/2, p3/2, p1/2).

Conclusions:

• Equation-of-motion coupled-cluster approximations provide a computationally feasible and accurate alternative to traditional configuration-interaction methods for studying nuclear structure.
• These CC methods are particularly effective for systems with a large number of correlated fermions, such as medium-mass nuclei.
• The findings highlight the practical applicability of EOM-CC for advancing nuclear structure theory and understanding nuclear properties.