https://psicode.org/psi4manual/master/introduction.html#overview
PSI4 provides a wide variety of quantum chemical methods using state-of-the-art numerical methods and algorithms. Several parts of the code feature shared-memory parallelization to run efficiently on multi-core machines (see Sec. Threading). An advanced parser written in Python allows the user input to have a very simple style for routine computations, but it can also automate very complex tasks with ease.
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, and high-order coupled-cluster theory.
In this section, we provide an overview of some of the features of PSI4 along with the prerequisite steps for running calculations. Sec. Tutorial provides a brief tutorial to help new users get started. Section Psithon offers further details into the structure of PSI4 input files and how Python can be mixed with quantum chemistry directives in PSI4. Section Psithon Functions provides more detail on the Python functions provided by PSI4 and discusses some of the higher-level functions such as counterpoise correction, complete-basis-set extrapolation, and running computations on an entire database of molecules at a time. Later sections deal with the different types of computations which can be done using PSI4 (e.g., Hartree–Fock, MP2, coupled-cluster) and general procedures such as geometry optimization and vibrational frequency analysis. The Appendices include a complete description of all possible input keywords for each module, as well as tables of available basis sets and a listing of the sample input files available under psi4/samples. The user is urged to examine this directory of sample inputs, as most common types of computations are represented there. For the latest PSI4 documentation, check www.psicode.org.