Advertised Version: 1.0 Continuous Version: 1.0 Release Date: 4 Jul 2016 Documentation: http://psicode.org/psi4manual/1.0.0/ Availability: Public, GitHub source, CMake build, Conda binary installers

Psi4 is, in many ways, a whole new package compared to Psi3. While some libraries and modules remain the same, the majority of the code has been rewritten from scratch based on a powerful set of new libraries written in C++. A totally new Python front-end makes Psi4 incredibly user-friendly and automates many common tasks such as basis set extrapolation, composite methods, running the same computation on every molecule in a test set, etc. Density-functional theory, absent in Psi3, is quite efficient in Psi4, with many functionals available. Density fitting is ubiquitous in Psi4, leading to some of the most efficient MP2 and CCSD(T) code available. Psi4 also introduces extensive, powerful features for energy component analysis of non-covalent interactions via symmetry-adapted perturbation theory. Orbital-optimized versions of perturbation theory and coupled-cluster methods, and their analytic gradients, have also been added. Through external libraries, Psi4 gains access to implicit solvent (PCM) capabilities, density-matrix renormalization group CI, effective fragment potentials, Grimme dispersion corrections, Stone’s distributed multipole analysis, and high-order coupled-cluster theory.