Application of the DIIS technique to improve the convergence properties of the SCF-MI algorithm

The direct inversion in the iterative subspace (DIIS) technique of Pulay is incorporated within the self consistent field for molecular interactions (SCF-MI) algorithm. It is shown to improve significantly the convergence rate of the conventional SCF-MI iteration scheme. The implemented procedure demonstrates stability and rate of convergence similar to that of standard DIIS-based codes. Its performance is compared to that of the level-shifting technique as applied within the SCF-MI algorithm on test examples involving molecular complexes and standard basis sets.