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Ohio State University

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A new type of approximate force field for the vibrational analysis of polyatomic molecules is being explored as tested in our laboratory. This force field, called the adjusted valence force field (AVFF), is a modified form of the generalized valence force field (GVFF), wherein the interactions between valence coordinates (bond-bond, bond-angle and angle-angle) about a particular nonterminal atom are treated as a weighted fraction of the average value of the corresponding valence force constants. For example, the interaction between between a bond i and another bond $k_{ij}$, where i and j share a common nonterminal atom, is represented by $k_{ij}=f (K_i+K_j)/2; K_i$ and $K_{j}$ being the valence force constants for bonds i and j, respectively, and f the weighting factor (referred to as the interaction correlation factor). Similar relationships are developed for bond-angle and angle-angle interactions in terms of the valence force constants and the interaction correlation factor (ICF), with all of the connected interactions about particular nonterminal atom being scaled by the same ICF. Although some physical basis for the AVFF concept can be seen in prior discussions of vibrational anlaysis methods, no evidence has been found of prior comprehensive applications of the concept. Tetrahedral molecules of the $AB_{4}$ type have been treated using a three-parameter AVFF which yields a fitting capability that is comparable in accuracy to a three-parameter orbital valence force field (OVFF and is generally superior to a three-parameter Urey-Bradley force field (UBFF). An expanded version of the AVFF incorporating a term for nonconnected interactions, has shown impressive agreement with the accurately determine potential functions derived for molecules that have well established harmonic frequencies (including those for isotopic molecules) and Coriolis constants, such as $CF_{4}, SiF_{4}$, and ONF. The application of the AVFF to these molecules and to several large molecules with multiple nonterminal atoms will be presented and discussed.


$^{\ast}$ Work performed under the auspices of the U.S. Department of Energy.
Author Institution: Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois