Psi is an ab initio computational chemistry package originally written by the research group of Henry F. Schaefer, III (University of Georgia).

[2][3] The major part of the program is written in C++, while Python API is also available, which allows users to perform complex computations or automate tasks easily.

The basic capabilities of Psi are concentrated around the following methods[2] of quantum chemistry: Several methods are available for computing excited electronic states, including configuration interaction singles (CIS), the random phase approximation (RPA), time-dependent density functional theory (TD-DFT), and equation-of-motion coupled cluster (EOM-CCSD).

[2] Psi4 has introduced the density-fitting approximation in many portions of the code, leading to faster computations and reduced I/O requirements.

[7] In Psi4 1.4,[6] the program was adapted to facilitate high-throughput workflows and can be connected to BrianQC to speed up calculations for Hartree-Fock and Density functional theory methods.