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% History of Psi
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% Daniel Crawford, 24 January, 1996
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The PSI suite of {\em ab initio} quantum chemistry programs is the result
of an ongoing attempt by a cadre of graduate students, postdoctoral
associates, and professors to produce code that is efficient but also
easy to extend to new theoretical methods.  Significant effort has been
devoted to the development of libraries which are robust and easy to use.
Some of the earliest contributions to what is now referred to as ``PSI''
include a direct configuration interaction (CI) program (Robert Lucchese,
1976, now at Texas A\&M), the well-known graphical unitary group CI program
(Bernie Brooks, 1977-78, now at N.I.H.), and the original integrals code
(Russ Pitzer, 1978, now at Ohio State).  From 1978-1987, the package was
know as the {\tt BERKELEY} suite, and after the Schaefer group moved to the
Center for Computational Quantum Chemistry at the University of Georgia,
the package was renamed {\tt PSI}.  Thanks primarily to the efforts of Curt
Janssen (Sandia Labs, Livermore) and Ed Seidl (LLNL), the package was
ported to UNIX systems, and substantially improved with new input formats
and a C-based I/O system.

Beginning in 1999, an extensive effort was begun to develop \PSIthree\ ---
a {\tt PSI} suite with a completely new face.  All of the code is now in
C and C++, including new integral/derivative integral, coupled cluster,
and CI codes.  In addition, new I/O libraries have been added, as well as
an improved checkpoint file structure and greater automation of typical
tasks such as geometry optimization and frequency analysis.  The package
has the capability to determine wavefunctions, energies, analytic gradients,
and various molecular properties based on a variety of theories, including
spin-restricted, spin-unrestricted, and restricted open-shell Hartree-Fock
(RHF, UHF, and ROHF); configuration interaction (CI) (including a variety
of multireference CI's and full CI); coupled-cluster (CC) including CC with
variationaly optimized orbitals; second-order M{\o}ller-Plesset perturbation
theory (MPPT) including explicitly correlated second-order M{\o}ller-Plesset
energy (MP2-R12); and complete-active-space self-consistent field (CASSCF)
theory.
