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BIGSTICK: A flexible configuration-interaction shell-model code (updated)

Calvin W. Johnson, W. Erich Ormand, Kenneth S. McElvain, Ryan Zbikowski, Hongzhang Shan

TL;DR

BIGSTICK advances the configuration-interaction shell-model by delivering a flexible, parallelized framework capable of handling two- and three-body forces through a factorized, on-the-fly Hamiltonian. It unifies M- and J-scheme bases via quantum-number factorization, enabling large-scale no-core and valence-space calculations with efficient Lanczos solvers and scalable MPI/OpenMP parallelism. The manual details input formats, truncation schemes, and a comprehensive set of runtime and post-processing options, including one- and two-body densities, strength functions, and Green's functions, supported by post-processing tools like RHODIUM and tracer. The approach supports both phenomenological and ab initio calculations, with robust diagnostics, memory-management strategies, and a clear pathway for users to generate, validate, and interpret spectra and transition properties across diverse many-fermion systems. The combination of factorization, scalable solvers, and flexible input/output makes BIGSTICK a practical and extensible tool for nuclear structure and related many-body problems, from laptop-scale tests to leadership-class HPC runs.

Abstract

We present BIGSTICK, a flexible configuration-interaction open-source shell-model code for the many-fermion problem. Written mostly in Fortran 90 with some later extensions, BIGSTICK utilizes a factorized on-the-fly algorithm for computing many-body matrix elements, and has both MPI (distributed memory) and OpenMP (shared memory) parallelization, and can run on platforms ranging from laptops to the largest parallel supercomputers. It uses a flexible yet efficient many-body truncation scheme, and reads input files in multiple formats, allowing one to tackle both phenomenological (major valence shell space) and ab initio (the so-called no-core shell model) calculations. BIGSTICK can generate energy spectra, static and transition one-body densities, and expectation values of scalar operators. Using the built-in Lanczos algorithm one can compute transition probability distributions and decompose wave functions into components defined by group theory. This manual provides a general guide to compiling and running BIGSTICK, which comes with numerous sample input files, as well as some of the basic theory underlying the code. Updated November 2025 to version 8.0.0

BIGSTICK: A flexible configuration-interaction shell-model code (updated)

TL;DR

BIGSTICK advances the configuration-interaction shell-model by delivering a flexible, parallelized framework capable of handling two- and three-body forces through a factorized, on-the-fly Hamiltonian. It unifies M- and J-scheme bases via quantum-number factorization, enabling large-scale no-core and valence-space calculations with efficient Lanczos solvers and scalable MPI/OpenMP parallelism. The manual details input formats, truncation schemes, and a comprehensive set of runtime and post-processing options, including one- and two-body densities, strength functions, and Green's functions, supported by post-processing tools like RHODIUM and tracer. The approach supports both phenomenological and ab initio calculations, with robust diagnostics, memory-management strategies, and a clear pathway for users to generate, validate, and interpret spectra and transition properties across diverse many-fermion systems. The combination of factorization, scalable solvers, and flexible input/output makes BIGSTICK a practical and extensible tool for nuclear structure and related many-body problems, from laptop-scale tests to leadership-class HPC runs.

Abstract

We present BIGSTICK, a flexible configuration-interaction open-source shell-model code for the many-fermion problem. Written mostly in Fortran 90 with some later extensions, BIGSTICK utilizes a factorized on-the-fly algorithm for computing many-body matrix elements, and has both MPI (distributed memory) and OpenMP (shared memory) parallelization, and can run on platforms ranging from laptops to the largest parallel supercomputers. It uses a flexible yet efficient many-body truncation scheme, and reads input files in multiple formats, allowing one to tackle both phenomenological (major valence shell space) and ab initio (the so-called no-core shell model) calculations. BIGSTICK can generate energy spectra, static and transition one-body densities, and expectation values of scalar operators. Using the built-in Lanczos algorithm one can compute transition probability distributions and decompose wave functions into components defined by group theory. This manual provides a general guide to compiling and running BIGSTICK, which comes with numerous sample input files, as well as some of the basic theory underlying the code. Updated November 2025 to version 8.0.0

Paper Structure

This paper contains 128 sections, 132 equations, 3 figures, 3 tables.

Table of Contents

  1. Introduction
  2. Expectations of users
  3. How to cite and copyright notices/licenses
  4. LAPACK copyright notice
  5. Reporting bugs and other issues
  6. What's new in each version
  7. A brief history of BIGSTICK, and acknowledgements
  8. Available post-processing codes
  9. This version
  10. How we solve the many-body problem
  11. Matrix formulation of the Schrödinger equation
  12. Representation of the basis
  13. Use of quantum numbers: factorization of the basis
  14. The Lanczos algorithm and computational cost
  15. Representation of the Hamiltonian
  16. ...and 113 more sections

Figures (3)

  • Figure 1: Segregation of single-particle space. 'Inert core' has all states filled. 'Excluded' disallows any occupied states. 'All valence' can have states up to the number of valence particles filled, while 'Limited valence' can only have fewer states filled (e.g. one, two, three...). See text for discussion. Figure taken from BIGSTICK.
  • Figure 2: Illustration of how transition strengths evolve with increasing number of Lanczos iterations. In this example, the operator $\vec{\sigma} \tau_0$ was applied to the ground state of $^{28}$Si, calculated in the sd shell with the USDB interaction.
  • Figure 3: Running sums of the strengths shown in Fig. \ref{['si28gamow']}.