A Theoretical Investigation of the Geometries, Vibrational Frequencies, and Binding Energies of Several Alkali Halide Dimers

Results are presented from ab initio calculations on the symmetrical alkali halide dimers made up of Li, Na, K, F, and Cl. We examine the sensitivity of representative monomer and dimer geometries to the variation of the basis set with and without polarization and diffuse functions. The geometries are then compared with available experimental results. We have also calculated vibrational frequencies at the restricted Hartree–Fock level and examined the changes in geometry brought about by correlation using second‐order Møller–Plesset perturbation theory. It is found that Hartree–Fock theory in a modest basis set with diffuse and polarization functions yields results comparable to much larger sp basis sets and that the theoretical results are in good agreement with the experimental results for the Li and Na dimers. Our best results for the K‐containing species tend to have bond lengths that are too long for the monomers and this error is carried over for the dimers. We also find a nearly uniform expansion of the M–X bond length in proceeding from monomer to dimer of 0.16±0.03 Å, independent of the alkali or halide involved. The calculated dimer dissociation energies are in excellent agreement with experiment. Inclusion of correlation appears to have a minimal effect on the computed geometries and a modest effect on the binding energies. The vibrational frequencies for the monomers are in excellent agreement with experimental gas‐phase results and reasonable agreement is obtained with the available experimental frequencies for the dimers. Finally, a reanalysis of the electron diffraction data for Na2F2 in light of the current ab initio results leads to a significant change in the experimental value of the bond angle