Composing and decomposing quantum chemistry software: Adventures with Psi4 and QCArchive

The Psi4 quantum chemistry (QC) program maintains a full-featured core but also has accumulated interfaces with over a dozen specialist community libraries. Recently, Psi4 has added to its ecosystem by divesting some of its post-processing and interface features into independent projects knitted together by QCSchema, a collection of JSON-based communication structures governed by Pydantic, and run by QCEngine, an I/O standardizer for two dozen chemistry projects. QCSchema and QCEngine are part of the Quantum Chemistry Archive (QCA), an umbrella project by the Molecular Sciences Software Institute (MolSSI) to accumulate and serve high-volume quantum chemical results for the community. Building upon QCA, Psi4's newly reworked outer Python layer facilitates high-throughput computing of naturally parallel procedures such as composite methods or many-body routines with minimal changes to the input. Capabilities to call Psi4 and other QC programs through increasingly uniform input suitable for software generation will also be presented.