Psi4 is a free and open-source ab initio electronic structure program providing Hartree-Fock, density functional theory, many-body perturbation theory, configuration interaction, density cumulant theory, symmetry-adapted perturbation theory, and coupled-cluster theory. Most of the methods are quite efficient thanks to density fitting and multi-core parallelism. The program is a hybrid of C++ and Python, and calculations may be run with very simple text files or using the Python API, facilitating post-processing and complex workflows; method developers also have access to most of Psi4's core functionality via Python. Job specification may be passed using The Molecular Sciences Software Institute (MolSSI) QCSchema data format, facilitating interoperability. A rewrite of our top-level computation driver, and concomitant adoption of the MolSSI QCArchive Infrastructure project, make the latest version of Psi4 well suited to distributed computation of large numbers of independent tasks. The project has fostered the development of independent software components that may be reused in other quantum chemistry programs. File list (2) download file view on ChemRxiv psi4.pdf (4.37 MiB) download file view on ChemRxiv supplementary_material.pdf (297.86 KiB)

Stabilized Criegee intermediates (SCI) are formed during the ozonolysis of unsaturated hydrocarbons and have been implicated in the formation of hydroxyl radicals and aerosols. Previous theoretical research [S. Jørgenson and A. Gross, J. Phys. Chem. A, 2009, 113, 10284-10290] computed the rate constants for addition of ammonia to simple SCIs, but reported a wide distribution of quantum chemical energies, depending on the basis set used. We report optimized geometries for these reactions at the CCSD(T)/ANO2 and CCSD(T)/ANO1 levels, and CCSD(T)/CBS energies with perturbative quadruples corrections. We find the inclusion of perturbative quadruples corrections elevates the energy of the transition state by 0.76-0.88 kcal mol relative to the reactants, which qualitatively changes the reaction surface. We calculate rate constants and find that Jørgenson and Gross previously overestimated the rate constants for ammonia addition to SCIs, but were within an order of magnitude. This supports the previous conclusion of Vereecken et al. [L. Vereecken, H. Harder and A. Novelli, Phys. Chem. Chem. Phys., 2012, 14, 14682-14695] that ammonia addition to SCIs is a negligible sink of Criegee intermediates.

Community efforts in the computational molecular sciences (CMS) are evolving toward modular, open, and interoperable interfaces that work with existing community codes to provide more functionality and composability than could be achieved with a single program. The Quantum Chemistry Common Driver and Databases (QCDB) project provides such capability through an application programming interface (API) that facilitates interoperability across multiple quantum chemistry software packages. In tandem with the Molecular Sciences Software Institute and their Quantum Chemistry Archive ecosystem, the unique functionalities of several CMS programs are integrated, including CFOUR, GAMESS, NWChem, OpenMM, Psi4, Qcore, TeraChem, and Turbomole, to provide common computational functions, i.e., energy, gradient, and Hessian computations as well as molecular properties such as atomic charges and vibrational frequency analysis. Both standard users and power users benefit from adopting these APIs as they lower the language barrier of input styles and enable a standard layout of variables and data. These designs allow end-to-end interoperable programming of complex computations and provide best practices options by default.