The Urey — Bradley Force Field: Its Significance and Application

Abstract

One of the most fundamental problems in normal coordinate analysis is the selection of a suitable potential field to express the interatomic forces in a molecule. Although the generalized valence force (GVF) field is widely used by many investigators, it encounters serious difficulties in complex molecules since the number of force constants exceeds the number of observed normal vibrations. In order to circumvent this difficulty, Shimanouchi introduced the Urey-Bradley force (UBF) field which consists of stretching and bending force constants, as well as repulsive force constants between nonbonded atoms. The number of force constants in the UBF field is much smaller than that in the GVF field. In addition, the UBF field has the advantages that the force constants have a clearer physical meaning than those of the GVF field, and they are transferable from molecule to molecule. The UBF field, however, does not include any interaction terms between non-neighboring stretching vibrations and between bending vibrations. In some molecules, ignorance of these terms causes difficulties in adjusting force constants to fit the observed frequencies. Recently, a number of attempts have been made to improve the UBF field, and these will be reviewed briefly.