Alkali halide molecules: Configurations and molecular characteristics of dimers and trimers

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Abstract
The equilibrium configurations, equilibrium distances, binding energies, and vibrational frequencies have been calculated for dimer and trimer molecules of the alkali halides. The treatment is based on an earlier application of a simple shell model to monomer molecules and crystals of the alkali halides in which all characteristic interaction parameters have been assigned. Thus, there are no adjustable parameters in the present calculation of the dimer and trimer characteristics. The stable dimer configuration was found to be the rhombus with the halogen–metal–halogen angle varying systematically from greater than 90° to less than 90° as (r −−r +), the difference in the ionic radii, varies from positive to negative [i.e., at (r −−r +) ?0 the configuration is essentially a perfect square, e.g., Na2F2, Rb2Cl2]. The linear dimers are unstable relative to the rhombus. The perfect hexagon was found to be the stable configuration of the trimer molecules. There is good agreement with the meager experimental data available.