Molecular dynamics studies of the equilibrium and saddle-point geometries of MXn molecules (n = 2-7)

Abstract

Two nuclear potential functions that have the property of invariance to the operations of the permutation group of nuclei in molecules of the general formula MX/sub n/, n = 2-7, are described. Such potential functions allow equivalent isomers to have equal energies so that various statistical mechanical properties can be simply determined. The first contains two-center interactions between pairs of peripheral atoms, and the second function contains three-center interactions. Equations are given which define both the functions in terms of k and Q, force constants, /Delta/r/sub /alpha//mu//, the change in bond length between the x atom /alpha/ and the central atom /mu/, and /theta/, the angle subtended at the central atom X by atoms /sup /alpha//mu//beta///alpha/ and /beta/ for which the preferred value is /pi/. The force fields derived from these two potential functions have been used to determine the equilibrium and saddle-point geometries of the series of molecules MX/sub n/, n = 2-7. These fields predict equivalent equilibrium and saddle-point geometries for MX/sub 2/ through MX/sub 6/ but not for MX/sub 7/. In this case, the first function gives a D/sub 5h/ but equilibrium geometry with two close-lying saddle-point geometries (the first of symmetry C/sub 2v/, a trigonal prismmore » capped on a square face; the other C/sub 3v/, an octahedron capped on a face). The second potential function gives an equilibrium geometry with symmetry C/sub 1/ and the three geometries above as close-lying saddle-point ones. In addition, the dynamic behavior of MX/sub 5/ and MX/sub 7/ molecules follows as a natural consequence of these force fields in contrast to the relative rigidity of the others which belong to the crystallographic point groups. 41 refs., 2 figs., 3 tabs.« less