The molecular modeling methods have found wide acceptance in the fields of organic chemistry and biochemistry. This study introduces a new algorithm to overcome the special problems encountered when extending standard force fields to metal-organic and inorganic complexes. The extension can easily be incorporated in common modeling programs. The application to the complexes of Cu(II) in square-planar coordination validates the approach. A general parametrization for Cu(II) consistent with the TRIPOS 5.2 force field is developed on the basis of the X-ray structures of 27 complexes with nitrogen- and oxygen-coordinated copper. The metal environment can be calculated to better than 2 pm for the bond lengths and 2-degrees for the bond angles. Some larger deviations for the torsion angles can be attributed to the intermolecular interactions in the crystal.