LiF clusters in the polarization model (original) (raw)

1982, Journal of Molecular Structure: THEOCHEM

The recent report of Swepston et al. [l] of high quality ab initio quantum mechanical computations on clusters of LiF molecules prompted these computations on LiF clusters based on the polarization model for water and its ionic dissociation products [ 21. A polarization model for solid LiF would allow simulation of this solid and of the solution process wherein a crystal of LiF in a sample of water is allowed (presumably under molecular dynamic conditions) to dissolve. These computations are possible due to the fact that the polarization model has been excited to fluoride in its interactions with protons [3] and with water [4] and to the Li+ cation [ 51 in its interactions with water. Further, the Li-F system has been calibrated against the results of Brumer and Karplus [6] in the context of the polarization model. In another report based on the same calibration, the process of bringing an Li+ cation (tetrahydrated) into collision with a F-anion (also tetrahydrated) has been explored [ 71. Here, an investigation of the minimum energy structures found by the polarization model for (LiF), clusters, for n = 2, 3,4, and 8, is reported. In these computations, there is no K(r) or L(r) function for Li'. The polarization energy of the monomer of LiF,-185.0 kcal mol-', has been adjusted to fit the Brummer and Karplus value. Table 1 shows the values of the energies which have been achieved for various (LiF),. As can be seen, within reason all the structures generated are non-polar. There is no doubt that our results mimic in several ways the results of the ab initio computations. The LiF bond length in the cyclic trimer is reproduced almost perfectly (1.58 vs. 1.687 A from the ab initio work). On the other hand, the energy difference on dimerizing LiF, relative to two isolated molecules, is reported as-76.1 kcal mol-', while we obtain only-41.5 kcal mol-'. Furthermore, the quantum mechanical calculations obtain 1.71 K for r(LiF) while we obtain 1.58A in the dimer. On trimerizing, the relative error is even worse, with the ab initio calculations reporting-150.8 kcal mol-', while we obtain only-89.7 kcal mol-'. Our cyclic quadrimer relaxes from the embryonic NaCl starting structure to the planar, cyclic zero dipole moment structure. In packing two such cyclic quadrimers in parallel planes, two structures emerged. The first