AB INITIO CALCULATION OF ANHARMONIC AND HARMONIC FORCE CONSTANTS
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Date
1976
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Ohio State University
Abstract
First, a short review of the ab initio harmonic force field calculations is given, with emphasis on the Hartree-Fock (H-F) method. The main terms (diagonal force constants in valence coordinates) show considerable systematic deviation in the H-F method. Although empirical corrections are possible, experimental accuracy cannot be achieved. On the other hand, for the smaller interaction constants the theoretical and experimental results are of comparable accuracy. Therefore, a combined approach is suggested, utilizing both kinds of information. The wrong dissociation behavior of the H-F diatomic potential curves has long discouraged the calculation of higher force constants from H-F wavefunctions. However, recent results show conclusively that correlation energy as a function of nuclear coordinates is dominated by the linear term. This suggests and calculations on $N_{2}, H_{2}, OH, HCN, C_{2} H_{2}$ and $CH_{4}$ show that the H-F method is very successful in predicting cubic force constants and performs well even for quartics if the experimental equillibrium geometry is used as reference geometry, A justification of this procedure is given. Accuracy of cubic and quartic interactions is discussed.
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Author Institution: University of Texas