Estimation of an Urey-Bradley force field from ab initio calculations

Urey-Bradley (UB) type force constants have been derived from the Cartesian force constant as obtained through ab initio calculations. The matrix of the force constants is expressed in the internal coordinate space and its elements are scaled so that the theoretical vibrational frequencies fit the experimental ones. Predicted UB type force constants are then extracted from this scaled internal force constant matrix using a Z matrix representing internal coordinate interactions in the UB space. We also include a Jacobian matrix determination into the SPASIBA program to help in refining the predicted force constants. These force constants must lead to a good vibrational force field together with an accurate reproduction of the equilibrium geometries. This process was applied to H2O2, H2S2, NH2OH, N2H4 and CH3OH. (C) 1997 Elsevier Science B.V. (C) 1997 Elsevier Science B.V.