Abstract: Anew method is presented to perform high accurate quantum chemical calculations in a local area of large systems. The system is partitioned into the active and surrounding areas. Acomparatively rough calculation is performed for the system to produce the approximate density matrix and it is transformed into a direct sum of the submatrices related to the active and surrounding areas, respectively, which involve integer electrons and form relatively independent subsystems with saturated chemical bonding. Then the surrounding area is frozen, and high accurate quantum chemical calculations are performed in the active area with the action of the potential produced by the charge in the surrounding area being taken into account. The calculations have been performed for a series oforganic molecules by use of the proposed method, and the data of Mulliken charges, bond lengths, bond dissociation energies, the first ionization energies and electronegativities are obtained, which are in satisfactory agreement with those obtained from the high accurate calculations of the whole systems, while the computational effort of the former is considerably reduced comparing to the latter. It can be concluded that the proposed method is practical and valuable.

Key words: Quantum chemistry, Computational method, Density matrix, Density functional calculation, High accurate calculation in local areas