VALENCE BOND CONCEPTS APPLIED TO THE MOLECULAR MECHANICS DESCRIPTION OF MOLECULAR SHAPES .1. APPLICATION TO NONHYPERVALENT MOLECULES OF THE P-BLOCK

Most molecular mechanics methods attempt to describe accurate potential energy surfaces by using a variant of the general valence force field (commonly using the diagonal terms, only) and a large number of parameters. However, these simple force fields are not accurate outside the proximity of the energetic minima and often are difficult to apply to the different shapes and higher coordination numbers of, for example, transition metal complexes. As the application of molecular mechanics methods is extended to collections of atoms that span the entire periodic table, the requisite number of parameters rapidly becomes unmanageable. For this work, we adopt readily derived hybrid orbital strength functions as the basis for a molecular mechanics expression of molecular shapes. These functions are suitable for accurately describing the energetics of distorting bond angles not only around the energy minimum but also for very large distortions as well. The combination of these functions with simple valence bond ideas (such as Bent's rule) leads to a simple scheme for predicting molecular shapes. Structures and vibrational frequencies calculated by the VALBOND program agree well with experimental data for a variety of molecules from the main group of the periodic table. Overall the qualities of the results are similar to those of other popular force fields (such as MM3) despite the use of fewer angular parameters.