Molecular geometry anharmonic effects

Usually the harmonic approximation with some anharmonicity corrections suffice to describe the energy dependence on the interatomic separations of the bonded atoms in the vicinity of the minimum PES. Nevertheless, sometimes, when it attempts either to simultaneously reproduce molecular geometries and the heats formation or simply to cover a wider range of molecular geometries where anharmonicity effects become more pronounced, the bond stretching energy terms are taken in the form ... 

Because of their importance to nucleation kinetics, there have been a number of attempts to calculate free energies of formation of clusters theoretically. The most important approaches for the current discussion are harmonic models, " Monte Carlo studies, and molecular dynamics calcula-tions. In the harmonic model the cluster is assumed to be composed of constituent atoms with harmonic intermolecular forces. The most recent calculations, which use the harmonic model, have taken the geometries of the clusters to be those determined by the minimum in the two-body additive Lennard-Jones potential surface. The oscillator frequencies have been obtained by diagonalizing the Lennard-Jones force constant matrix. In the harmonic model the translational and rotational modes of the clusters are treated classically, and the vibrational modes are treated quantum mechanically. The harmonic models work best at low temjjeratures where anharmonic-ity effects are least important and the system is dominated by a single structure. 

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