<scp>Psi4</scp> 1.1: An Open-Source Electronic Structure Program Emphasizing Automation, Advanced Libraries, and Interoperability

TLDR

Psi4 is an ab initio electronic structure program offering Hartree‑Fock, DFT, CI, and coupled‑cluster methods. The 1.1 release aims to automate complex tasks such as geometry optimization with complete‑basis‑set extrapolation or focal‑point methods. The release rewrites the top‑level code as a Python module, integrates libraries for density fitting, Cholesky decomposition, and Laplace denominators, rewrites the build system for interoperability, and adds numerous advanced methods including symmetry‑adapted perturbation theory, density‑fitted coupled‑cluster, and relativistic corrections.

Abstract

Psi4 is an ab initio electronic structure program providing methods such as Hartree-Fock, density functional theory, configuration interaction, and coupled-cluster theory. The 1.1 release represents a major update meant to automate complex tasks, such as geometry optimization using complete-basis-set extrapolation or focal-point methods. Conversion of the top-level code to a Python module means that Psi4 can now be used in complex workflows alongside other Python tools. Several new features have been added with the aid of libraries providing easy access to techniques such as density fitting, Cholesky decomposition, and Laplace denominators. The build system has been completely rewritten to simplify interoperability with independent, reusable software components for quantum chemistry. Finally, a wide range of new theoretical methods and analyses have been added to the code base, including functional-group and open-shell symmetry adapted perturbation theory, density-fitted coupled cluster with frozen natural orbitals, orbital-optimized perturbation and coupled-cluster methods (e.g., OO-MP2 and OO-LCCD), density-fitted multiconfigurational self-consistent field, density cumulant functional theory, algebraic-diagrammatic construction excited states, improvements to the geometry optimizer, and the "X2C" approach to relativistic corrections, among many other improvements.