Chronus Quantum 1.0: A unified framework for relativistic, time-dependent, and multicomponent electronic structure theory

We present Chronus Quantum (ChronusQ) 1.0, an open-source software platform for modern electronic structure theory with particular emphasis on the molecular Dirac–Coulomb–Breit Hamiltonian, relativistic many-body methods, multicomponent quantum chemistry, and time-dependent phenomena. Rather than serving as another general-purpose quantum chemistry package, ChronusQ is designed to develop and disseminate new theoretical methodologies through reusable, interoperable, and extensible software frameworks. At the heart of the software is a unified relativistic infrastructure spanning the molecular Dirac–Coulomb–Breit Hamiltonian, exact two-component methods, atomic-orbital integral engines, Cholesky decomposition, finite-magnetic-field gauge-including atomic orbitals, and distributed tensor computations. Building upon these shared components, ChronusQ provides relativistic self-consistent-field and density functional theory, multiconfigurational methods based on the small-tensor-product distributed active space framework, relativistic coupled-cluster theory, nuclear–electronic orbital methods, and real-time quantum dynamics. This article presents the scientific capabilities, software architecture, and development philosophy of ChronusQ, illustrating how interoperable software infrastructures can accelerate the development of modern electronic structure methods while providing a flexible and sustainable platform for future advances in molecular quantum science.