ESTIMATION OF AN UREY-BRADLEY FORCE-FIELD FROM AB-INITIO CALCULATIONS

Urey-Bradley (UB) type force constants have been derived from the Cart esian force constant as obtained through ab initio calculations. The m atrix of the force constants is expressed in the internal coordinate s pace and its elements are scaled so that the theoretical vibrational f requencies fit the experimental ones. Predicted UB type force constant s are then extracted from this scaled internal force constant matrix u sing a Z matrix representing internal coordinate interactions in the U B space. We also include a Jacobian matrix determination into the SPAS IBA program to help in refining the predicted force constants. These f orce constants must lead to a good vibrational force field together wi th an accurate reproduction of the equilibrium geometries. This proces s was applied to H2O2, H2S2, NH2OH, N2H4 and CH3OH. (C) 1997 Elsevier Science B.V. (C) 1997 Elsevier Science B.V.