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

TL;DR

BIGSTICK is a versatile, open-source shell-model code capable of handling complex many-fermion problems with advanced parallelization and truncation schemes, suitable for both phenomenological and ab initio nuclear physics calculations.

Contribution

It introduces a flexible, efficient, and parallelized shell-model code with multiple input formats and advanced features like on-the-fly matrix element computation and wave function decomposition.

Findings

Supports large-scale energy spectrum calculations

Enables detailed wave function analysis with group theory

Runs efficiently on various computing platforms

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