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.
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.
W. Duch, and G. Diercksen. The Journal of Chemical Physics, 101 (4):
3018-3030(August 1994)Multi-reference form of the Davidson correction given here..