Structure and bonding in magnesium difluoride clusters: The MgF2 molecule

We have calculated the ground-state geometry, vibrational frequencies, and bonding properties of MgF2 at the Hartree-Fock (HF), second-order (MP2), an d fourth-order Moller-Plesset (MP4(SDTQ)) levels of calculation. Several hi gh-quality basis sets have been used, with special attention on the influen ce of polarization and diffuse functions on the above properties. The best HF and MP2 calculations predict that MgF2 is a linear molecule. MP2 and MP4 results are very similar. The MP2 symmetric (yl) and asymmetric (nu (3)) s tretching frequencies are about 5-7% smaller than the HF values and agree w ell with the observed data. The MP2 nu (2) (bending) frequency is close to that found in other ab initio calculations and the experimental gas-phase v alue but is 80 cm(-1) smaller than the value observed in the IR spectrum of MgF2 trapped in solid argon. Polarization functions shorten noticeably the magnesium-fluorine equilibrium distance and increase yl and nu (3) An atom s in molecules (AIM) analysis of the wave functions reveals that MgF2 is a highly ionic molecule, the net charge of Mg being about +1.8 e, and that mo st basis set effects are due to the poor convergence properties of the atom ic electron dipole moments. This suggests a polarizable ions model that is shown to account for the trends found in most of the properties studied. Th e origin of the bending problem in these compounds is traced back to the po larizability of the cation.