Abstract The CASSCF method of Roos, Taylor and Siegbahn has been implemented and extended. In addition to the super-Cl and two-step convergence method, a second-order convergence method is implemented. Further, CASSCF energy gradients are available. The CASSCF method is extended by second-order Epstein-Nesbet perturbation theory. Applications of these methods to the single-triplet splitting of CH 2 and also the determination of the fundamental frequencies of H 2O are reported.