This dialog gives the definition of each type of Urey-Bradley polyhedron. The central-ligand bonds must be defined in the Bonds dialog, and the ligand-central-ligand angles corresponding to each ligand-ligand "contact" must be defined in the Angles dialog. The ligand-ligand "contacts" or "bonds" are defined in the Ligand-Ligand Bond dialog, accessable from the main Urey-Bradley Data dialog.
The same Central atom type number should not be used for more than one polyhedron specification, even if coordination numbers are different. Thus it may be necessary to define more than one atom type (Atom Types dialog in the Input Menu) for a given chemical element. For example, aluminum in silicates often occurs in both tetrahedral and octahedral coordination, and carbon may occur in various coordinations in moderately complex molecules. Type numbers such as 13 and 113 for aluminum, and 6, 106 etc. for carbon can be assigned in such cases.
Coordination number may range from 2 to 8.
The Ligand atom types list does not require a type for each individual ligand, only the types necessary to include all ligands. For example, if all ligands are of type 8, the number 8 should be entered only once and other boxes should be blank.
The Kappa value is that in the approximation of Urey-Bradley forces by Shimanouchi. Once specifications have been entered, the force-constant value and least-squares flag for kappa may be changed in the overall Forces List, accessible from the Control Window. Kappa is used only for 4- and 6- coordinated polyhedra, which are assumed to be at least approximately tetrahedral and octahedral, respectively. If you are dealing with a 4- or 6- coordinated polyhedron which is not approximately tetrahedral or octahedral respectively (for example tetragonal planar), kappa should normally be set to zero. Setting kappa equal zero is equivalent to using a first-order approximation to the angular redundancies in the polyhedron, instead of second-order. Note, however, that in planar configurations the second-order terms vanish anyway.
Maximum distance. Any atoms bonded to the central atom at greater than the specified distance will not be included in U-B forces. If the value is 0.0, no test will be applied.