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Progress of Theoretical Physics Supplement

Monte Carlo Shell Model Calculations for Atomic Nuclei and Their Parallel Computing

Takaharu Otsuka, Noritaka Shimizu, Seigo Haruyama, Michio Honma, T. Mizusaki, Atsushi Taketani, Y. Utsuno, Yasushi Watanabe

Published Jan 1, 2000• 3 citations
Publisher

Abstract

The Quantum Monte Carlo Diagonalization method for solving quantum many-body problems has been proposed, and its applications to the nuclear shell model have been extensively developed. This application is called Monte Carlo Shell Model. The actual numerical calculations of Monte Carlo Shell Model can be carried out very efficiently by a massive parallel computer, and indeed we have introduced one called Alphleet to RIKEN recently. A brief sketch of the Monte Carlo Shell Model and its parallel computing are presented.

DOI
10.1143/ptps.138.24

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The Quantum Monte Carlo Diagonalization method for solving quantum many-body problems has been proposed, and its applications to the nuclear shell model have been extensively developed. This application is called Monte Carlo Shell Model. The actual numerical calculations of Monte Carlo Shell Model can be carried out very efficiently by a massive parallel computer, and indeed we have introduced one called Alphleet to RIKEN recently. A brief sketch of the Monte Carlo Shell Model and its parallel computing are presented.
The formulation and recent applications of the Quantum Monte Carlo diagonalization (QMCD) method are reported. The QMCD has been proposed for solving the quantum many-body inteacting systems, providing us with energy eigenvalues, transition matrix elements and wave functions. Its application to the nuclear shell model is referred to as the Monte Carlo Shell Model. By the Monte Carlo Shell Model calculations, the level structure of low-lying states can be studied with realistic interactions, providing a useful tool for nuclear spectroscopy. The Monte Carlo Shell Model has been applied to the study of a variety of nuclei, and can be characterized as the importance truncation scheme to the full diagonalization which is infeasible in many cases due to extremely large dimensions.
The shell model provides a powerful framework for nuclear structure calculations. The nucleons beyond an inert magic core are confined in a valence shell and interact through an effective two-body potential generally determined from the G-matrix method. However, the applicability of the shell model remains limited by the exponential growth of the many-body space with the number of valence nucleons and with the size of the one-body space. The quantum Monte Carlo (QMC) methods may thus be considered as a potentially attractive alternative to the direct diagonalization of the Hamiltonian matrix. The many-body problem is indeed reduced to a set of stochastic one-body problems describing independent particles in a fluctuating external field. Up to date, the QMC approaches of the shell model lead to the ground-state and the thermodynamic properties, but a detailed spectroscopy of nuclei is not reachable. Furthermore, they suffer from a common pathology of fermionic QMC methods, namely the sign/phase problem, which strongly contaminates the convergence of the calculation. In this context, we will present a new QMC method, providing the "yrast spectroscopy" of nuclei, and in which the sign/phase problem is managed through the constrained path approximation that is widely used in the ab initio nuclear calculations and in condensed matter physics.
We present novel Monte Carlo methods for treating the interacting shell model that allow exact calculations much larger than those heretofore possible. The two-body interaction is linearized by an auxiliary field; Monte Carlo evaluation of the resulting functional integral gives ground-state or thermal expectation values of few-body operators. The ``sign problem'' generic to quantum Monte Carlo calculations is absent in a number of cases. We discuss the favorable scaling of these methods with nucleon number and basis size and their suitability to parallel computation.
We propose a quantum Monte Carlo diagonalization method (QMCD) for solving the quantum many-body interacting systems. Not only the ground state but also low-lying excited states are obtained with their wavefunctions. Consequently the level structure of low-lying states can be studied with realistic interactions. The developments in the formulation of the QMCD are described with an illustrative and intuitive example. The QMCD is finally characterized as a `importance truncation' scheme to the shell model. After testing this method for Cr, we present first results for energy levels and E2 properties of Ge, indicating its large and -soft deformation. The doubly closed shell probability of Ni is shown to be only 49% in a full pf shell calculation, in contrast to the corresponding probability of Ca which reaches 86%. The prolate deformed excited band is obtained as a result of the QMCD calculation for non-yrast states, in a good agreement to recent experimental data. This band seems to have an SU (3) (-like) structure which was originally suggested by Elliott for sd-shell nuclei.
The formalism for the auxiliary-field Monte Carlo approach to the nuclear shell model is presented. The method is based on a linearization of the two-body part of the Hamiltonian in an imaginary-time propagator using the Hubbard-Stratonovich transformation. The foundation of the method, as applied to the nuclear many-body problem, is discussed. Topics presented in detail include: (1) the density-density formulation of the method, (2) computation of the overlaps, (3) the sign of the Monte Carlo weight function, (4) techniques for performing Monte Carlo sampling, and (5) the reconstruction of response functions from an imaginary-time auto-correlation function using MaxEnt techniques. Results obtained using schematic interactions, which have no sign problem, are presented to demonstrate the feasibility of the method, while an extrapolation method for realistic Hamiltonians is presented. In addition, applications at finite temperature are outlined.
The formalism for the auxiliary-field Monte Carlo approach to the nuclear shell model is presented.The method is based on a linearization of the two-body part of the Hamiltonian in an imaginary-time propagator using the Hubbard-Stratonovich transformation.The foundation of the method, as applied to the nuclear many-body problem, is discussed.Topics presented in detail include: (1) the density-density formulation of the method, (2) computation of the overlaps, (3) the sign of the Monte Carlo weight function, (4) techniques for performing Monte Carlo sampling, and ( 5) the reconstruction of response functions from an imaginary-time auto-correlation function using MaxEnt techniques.Results obtained using schematic interactions, which have no sign problem, are presented to demonstrate the feasibility of the method, while an extrapolation method for realistic Hamiltonians is presented.In addition, applications at finite temperature are outlined.