SEMIEMPIRICAL AND AB-INITIO CALCULATIONS OF THE FULL CONFIGURATION-INTERACTION USING ITERATED KRYLOVS SPACES

The algorithm proposed previously for calculating the full configurati on interaction using the variation matrix of the wave operator involve s the numerical solution of the corresponding incomplete eigenvalue pr oblem based on iterated Krylov's subspaces. In practice, that means us ing the multistep gradient method as a special version of the Lanczos method. The high efficiency of this algorithm, which can readily be us ed in personal computer calculations, is proved by particular ab initi o calculations of the full configuration interaction for the helium an d beryllium atoms as well as by semiempirical calculations of pi-shell s for naphthalene and diphenylene. The algorithm is of particular assi stance in obtaining numerous excited states, which are used for determ ining various spectral sums (polarizability, van der Waals interaction constants, and photoionization cross sections).