A page run by the center for computational studies of electronic structure and spectroscopy of open-shell and electronically excited species. Links to educational material on quantum chemistry, a forum, and free software are found here.
Molden is a package for displaying Molecular Density from the Ab Initio packages GAMESS-UK , GAMESS-US and GAUSSIAN and the Semi-Empirical packages Mopac/Ampac, it also supports a number of other programs via the Molden Format. Molden reads all the required information from the GAMESS / GAUSSIAN outputfile. Molden is capable of displaying Molecular Orbitals, the electron density and the Molecular minus Atomic density.
PyQuante is an open-source suite of programs for developing quantum chemistry methods. The program is written in the Python programming language, but has many "rate-determining" modules also written in C for speed.
COLUMBUS is a collection of programs for high-level ab initio molecular electronic structure calculations. The programs are designed primarily for extended multi-reference (MR) calculations on electronic ground and excited states of atoms and molecules.
cclib is an open source library, written in Python, for parsing and interpreting the results of computational chemistry packages. The current version parses output files from ADF, GAMESS (US), GAMESS-UK, Gaussian, Jaguar, Molpro, ORCA and Firefly.
This site contains details of various point-group symmetries, their inter-relations and specific information regarding dipole-transition selection rules.