Multicentred QM/QM methods for overlapping model systems

An improved, more general method for performing multicentred integrated QM/QM calculations is presented. The new approach allows the multicentred approximation to be extended to overlapping model systems, removing a significant limitation of the original approach. The usefulness and numerical accuracy of the equations presented are confirmed via some applications to dipole-dipole, charge-dipole and charge-charge complexes. The method performs well for all of these complexes, which range from very weakly to very strongly bound and in which non-additivity effects on interaction energies range from 0.2 to 17 kcal mol(-1).