STRUCTURAL, VIBRATIONAL AND ELECTRONIC-PROPERTIES OF A CRYSTALLINE HYDRATE FROM AB-INITIO PERIODIC HARTREE-FOCK CALCULATIONS

The hydrate crystal lithium hydroxide monohydrate LiOH.H2O has been st udied by ab initio periodic Hartree-Fock calculations. The influence o f the crystalline environment on the local molecular properties (molec ular geometry, atomic charges, electron density, molecular vibrations and deuterium quadrupole coupling constants) of the water molecule, th e lithium and hydroxide ions has been calculated. A number of crystall ine bulk properties are also presented, optimized crystalline structur e, lattice energy and electronic band structure. The optimized cell pa rameters from calculations with a large basis set of triple-zeta quali ty differ by only 1-3% from the experimental neutron-determined cell, whereas the STO-3g basis set performs poorly (differences of 5-10%). W ith the triple-zeta basis also the atomic positions and intermolecular distances agree very well with the experiment. The lattice energy dif fers by approximately 8% from the experimental value, and by at most 3 % when a density-functional electron correlation correction is applied . Large electron-density rearrangements occur in the water molecule an d in the hydrogen bond and are in qualitative and quantitative agreeme nt with experimental X-ray diffraction results. The quadrupole-couplin g constants of the water and hydroxide deuterium atoms are found to be very sensitive to the O-H bond length and are in good agreement with experimental values when the calculation is based on the experimental structure. The anharmonic O-H stretching vibrations in the crystal are presented and found to be very close to results from calculations on molecular clusters. The electronic band and density-of-states spectra are discussed. Model calculations on a hydrogen fluoride chain were us ed to rationalize the results.