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

Most molecular mechanics methods attempt to describe accurate potentia l energy surfaces by using a variant of the general valence force fiel d (commonly using the diagonal terms, only) and a large number of para meters. However, these simple force fields are not accurate outside th e proximity of the energetic minima and often are difficult to apply t o the different shapes and higher coordination numbers of, for example , transition metal complexes. As the application of molecular mechanic s methods is extended to collections of atoms that span the entire per iodic table, the requisite number of parameters rapidly becomes unmana geable. For this work, we adopt readily derived hybrid orbital strengt h functions as the basis for a molecular mechanics expression of molec ular shapes. These functions are suitable for accurately describing th e energetics of distorting bond angles not only around the energy mini mum but also for very large distortions as well. The combination of th ese functions with simple valence bond ideas (such as Bent's rule) lea ds to a simple scheme for predicting molecular shapes. Structures and vibrational frequencies calculated by the VALBOND program agree well w ith experimental data for a variety of molecules from the main group o f 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.