Siam Quantum 2: An open-source C toolbox for quantum modeling and electronic structure development

We present Siam Quantum 2, the next generation of the open-source C program originally published in 2012 for Hartree-Fock electronic structure calculations [KKU Journal of Science 12(3), 2012, DOI: 10.5481/KKUJGS.2012.12.3.2], developed in Thailand (historically known as Siam) as a platform for quantum chemistry research and education. Over the past decade, Siam Quantum has evolved into a modular toolbox supporting a wide range of quantum modeling capabilities, including density functional theory (DFT), second-order Møller-Plesset perturbation theory (MP2), analytic energy gradients, molecular geometry optimization, quantum molecular dynamics (QMD), and minimum energy crossing point (MECP) search. The present version introduces an extensible C architecture designed for method development, readability, and performance on multi-CPU environments. Each computational module is accompanied by human-readable Markdown documentation containing LaTeX-style equations and programming guides, bridging theoretical formulation and source code, and facilitating future computational development. The paper describes the program's design philosophy, theoretical framework, and implementation details. We also highlight studies by various research groups that have made use of Siam Quantum, illustrating how the software can be used for quantum modeling and electronic structure studies.