The restricted active space self-consistent-field method, implemented with a split graph unitary group approach

An MCSCF method based on a restricted active space (RAS) type wave function has been implemented. The RAS concept is an extension of the complete active space (CAS) formalism, where the active orbitals are partitioned into three subspaces: RAS1, which contains up to a given maximum number of holes; RAS2, where all possible distributions of electrons are allowed; and RAS3, which contains up to a given maximum number of electrons. A typical example of a RAS wave function is all single, double, etc. excitations with a CAS reference space. Spin-adapted configurations are used as the basis for the MC expansion. Vector coupling coefficients are generated by using a split graph variant of the unitary group approach, with the result that very large MC expansions (up to around 10⁶) can be treated, still keeping the number of coefficients explicitly computed small enough to be kept in central storage. The largest MCSCF calculation that has so far been performed with the new program is for a CAS wave function containing 716418 CSF's. The MCSCF optimization is performed using an approximate form of the super-CI method. A quasi-Newton update method is used as a convergence accelerator and results in much improved convergence in most applications.