Implementation of gradient‐optimization algorithms and force constant computations in BSSE‐free direct and conventional SCF approaches

A modification of the Roothaan equations was described in a previous work, which aimed to avoid the BSSE at the Hartree-Fock level of theory. The resulting scheme was called the self-consistent field for molecular interactions (SCF-MI) to underline its special usefulness in the computation of intermolecular interactions. The method provides a complete a priori elimination of the BSSE, while taking into account the natural nonorthogonality of the MOs of the two interacting fragments. Compatibility with the usual formulation of the analytic derivatives of the SCF energy is also guaranteed. This allowed the implementation of gradient-optimization algorithms and force constant matrix computations in both the direct and conventional SCF approaches. The SCF-MI method has been incorporated into the GAMESS-US package. Tests have been performed at the Department of Chemistry of the Iowa State University. Increases in the complication and computation time are minimal if compared to standard SCF codes and the method shows much less basis-set dependence in the predicted molecular properties.