JuliaCon 2026

Few-body systems physics is an intersection of many subfields of physics and chemistry. Its targets are various few-body systems across various scales, from hadrons and nuclei to atoms, molecules, and quantum dots. Ideally, researchers would like to calculate physical observables for arbitrary particle numbers, arbitrary interactions, and arbitrary quantum states. In practice, no single software package can satisfy all of these requirements, and researchers often develop self-tailored codebases for their specific problems.

To make this development culture scalable, we are building an open-source ecosystem as shared infrastructure that provides reusable building blocks and a flexible common interface in the Julia programming language. CI/CD with automated testing is a baseline requirement for maintainability and for accepting external contributions. Beyond its role in maintenance, testing also has a role in design, because the requirements for the system-level testability derive the architectural design.

Differential testing based on cross-method comparisons has been used in few-body calculations. For established benchmark problems, the final outputs can be validated using test oracles built from a database of results in the literature. However, for previously unexplored problems, there is no verified “correct answer”. This requirement motivates a common interface that allows multiple methods to operate on the same Hamiltonian.

The Rayleigh–Ritz method using explicitly correlated Gaussian (ECG) basis functions is widely used for solving the Schrödinger equation of quantum-mechanical few-body systems, while variational Monte Carlo (VMC) can calculate the energy for a given trial wave function. Currently, we are developing FewBodyECG.jl and MetropolisAlgorithm.jl (toward FewBodyVMC.jl) for cross-checking multiple methods. FewBodyHamiltonian.jl supports flexible construction of Hamiltonians as a common input for multiple methods. FewBodyDB.jl works as a test oracle database, and TwoBody.jl works as a prototype of FewBody.jl, the common interface of this JuliaFewBody ecosystem.

While we don't necessarily practice test-first or test-driven development, we find testability to be a compass for our overall architectural design and long-term development roadmap. We hope this ecosystem will open the gate to a "trading port" of computational methods.