Genäherte Lösung der Hartree-Fock-Gleichungen wechselwirkender Moleküle im Bereich der geringen Überlappung

A perturbational approach has been developed in order to determine the interaction energy of two molecules in the region of small overlap in an attempt to obtain some information upon reactivity. The total wavefunction of the interacting molecules is set up as a single determinantal wavefunction, whose basisfunctions are constructed as linear combinations of all occupied and unoccupied molecular orbitals. The coefficients are determined by an approximate solution of the Hartree-Fock-Roothaan equations of the total system. The SCF-iteration is performed analytically until all first and second order terms are included in the expression for the interaction energy. Due to the analytical procedure of the SCF-cycle the interaction energy contains no numerically iterated quantities. The interaction between a molecule with an open shell and a molecule with a closed shell is treated in the framework of the unrestricted Hartree-Fock-approximation. The interaction energy in the molecular orbital basis is transformed into an atomic orbital basis by using the LCAO-MO-approximation and introducing approximations for several integrals. Useful expressions for calculations are also derived, which contain only quantities of the isolated molecules, together with interatomic distances and overlap integrals. These formulae for the interaction energy are suitable for detailed interpretation of the different interaction terms and reveal the role of the interacting centres.