PSI is an ab initio computational chemistry package originally written by the research group of Henry F. Schaefer, III (University of Georgia). It performs high-accuracy quantum computations on small to medium-sized molecules.
PSI4 is the latest release of the program package - it is open source, released as free under the GPL through GitHub. Primary development of PSI4 is currently conducted by Daniel Crawford (Virginia Tech), David Sherrill (Georgia Tech), Justin Turney (University of Georgia), and Rollin King (Bethel University). PSI4 is available on Linux releases such as Fedora and Ubuntu.
Features
The basic capabilities of PSI are concentrated around the following methods of quantum chemistry:
Several methods are available for computing excited electronic states, including configuration interaction singles (CIS), the random phase approximation (RPA), and equation-of-motion coupled cluster (EOM-CCSD). PSI3 also includes the explicitly-correlated MP2-R12 method and the ability to compute the Born–Oppenheimer diagonal correction using configuration interaction wave functions.