Fragment: An Open-Source Framework for Multiscale Quantum Chemistry Based on Fragmentation

Fragment-based quantum chemistry offer a means to circumvent the nonlinear computational scaling of conventional electronic structure calculations by partitioning a large calculation into smaller subsystems, then considering the many-body interactions between them. Variants of this approach have been used to parameterize classical force fields and machine learning potentials, yet there are few readily available, purpose-built software packages to perform fragment-based calculations. To fill this void we introduce "FRAGMENT", an open-source software application that provides a tool for community validation of energy-based fragmentation methods, a platform for developing new approximations, and a framework for analyzing many-body interactions. FRAGMENT includes algorithms for automatic fragment generation and structure modification, and for distance- and energy-based screening of the requisite subsystems. Checkpointing, database management, and parallelization are handled internally and results are archived into a portable database format. Interfaces are provided to quantum chemistry engines including Q-Chem, PySCF, xTB, Orca, CP2K, MRCC, Psi4, NWChem, GAMESS, and MOPAC. Applications reported here demonstrate parallel efficiencies up to 96% on more than 1,000 processors but also showcase that the code can handle large-scale protein fragmentation using only workstation hardware. FRAGMENT conforms to modern software development best practices and is built upon well established technologies including Python, SQLite, and Ray. The source code is available under the Apache 2.0 license.