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PSI4 is an open-source suite of ab initio quantum chemistry programs developed by David Sherrill group and collaborators. The main strenghts of the code is in high-level correlated ab initio methods, MP2-R12 and SAPT theories. From the GaTech webpage "This package focuses on high-accuracy computations on small molecules, featuring methods such as CASSCF, multi-reference CI, and coupled cluster theory. The latest version, PSI4, has added very efficient new density-fitted code for Hartree-Fock, Density Functional Theory (DFT), and Second-Order Perturbation Theory (MP2). We have very efficient code to perform energy component analysis for intermolecular interactions via Symmetry Adapted Perturbation Theory (SAPT). PSI4 is also very user-friendly and makes it easy to automate complex tasks"