Application of numerical basis sets to hydrogen bonded systems: a density functional theory study

We have investigated and compared the ability of numerical and Gaussian-type basis sets to accurately describe the geometries and binding energies of a selection of hydrogen bonded systems that are well studied theoretically and experimentally. The numerical basis sets produced accurate results for geometric parameters but tended to overestimate binding energies. However, a comparison of the time taken to optimize phosphinic acid dimer, the largest complex considered in this study, shows that calculations using numerical basis sets offer a definitive advantage where geometry optimization of large systems is required.     

Spectroscopy of shear horizontal surface phonons by rotation-flip scattering of ortho-H2 molecules

Ortho-H2 molecules in the j=1 state are predicted, on the basis of a simple extension of the distorted wave approximation, to excite shear-horizontal (SH) polarized surface phonon modes, which are symmetry forbidden in inelastic He atom planar scattering. The symmetry breaking is provided by Deltam-rotational flip transitions which couple efficiently to the shear-horizontal modes. A confirmation is provided by the observation of SH surface phonons in phonon inelastic scattering of H2 from NaCl(001).     

Vibrational properties of C<Subscript>20</Subscript>-based solids

The phonon dispersion relations and IR spectrum of a C₂₀-based solid recently identified experimentally [Iqbal et al. , Eur. Phys. J. B 31, 509 (2003)] have been computed by density functional perturbation theory. Other competitive structures made by assembling C₂₀ clusters have been considered as well. In particular, we have computed the structure and the Raman spectra of two-dimensional polymeric phases of hydrogenated C₂₀ clusters which might be formed under different synthesis conditions. Fingerprints of the different phases have been identified in the vibrational spectra which could be used in the experimental search of C₂₀-based solids.Received: 29 September 2003, Published online: 15 March 2004PACS: 61.48. + c Fullerenes and fullerene-related materials - 63.20.-e Phonons in crystal lattices - 78.30.-j Infrared and Raman spectra     

The influence of lattice distortion on electron scattering in dilute alkali-alkali alloys

The electron scattering amplitudes of Na and Rb impurities in K are calculated starting from Shaw model potentials computed at the density of the host. Distortions induced by the impurities are explicitly accounted for, using lattice statics. Including this effect, we obtain better agreement with measured resistivities and predict an anisotropy in the electronic life time on the Fermi surface, which may be experimentally observable.     

The influence of covalency on anion-anion interionic potentials in NiO

Hartree-Fock self-consistent field cluster calculations were performed to investigate the short-range anion-anion interionic potential V₂₂ in NiO. The cluster consisted of two oxygen ions and ten surrounding Ni ions embedded in a matrix of point charges; the Ni²⁺ ions were represented by pseudopotentials. V₂₂ was extracted from calculations of the total cluster electronic energy as a function of the OO separation when one of the O ions is displaced from its lattice site towards the other O ion. The results indicate that covalency is mainly responsible for the anomalously large attractive terms in semiempirically derived OO interionic potentials.     

An AES study of the surface composition of cobalt ferrites

A series of cobalt ferrite samples, Co_1?yFe_{2 + y}O₄, where ?0.1 ? y ? 0.1, was examined by AES. A remarkable Fe surface enrichment was observed near the stoichiometric composition for y both positive and negative, and found to be in quite good agreement with theoretical calculations. The submission of samples to various treatments, at different temperatures and gas atmospheres, produced changes in the surface composition, in a fashion associated with the oxygen concentration both in gas and solid phase.     

Inelastic scattering of helium from bulk longitudinal acoustic phonons in LiF(001)

The analysis of some recent time-of-flight spectra reported by Doak for He inelastic scattering from LiF(001) along the 〈110〉 azimuth reveals important contributions from bulk longitudinal acoustic (LA) phonons. The large intensity due to LA phonons compared to that due to Rayleigh waves is shown to be the result of resonance enhancement. The independent determination of the coupling to LA phonons and to Rayleigh waves permits an estimate of the atom-surface, potential softness.     

Characterization of the NiNa6□Cl8 ordered phase by crystal field spectroscopy

We show that the formation of an ordered phase of transition metal impurities in ionic crystals can be monitored by means of crystal field spectroscopy of the d-d vibronic excitons. We present an optical absorption study of the Suzuki phase NiNa₆□Cl₈ in doped NaCl:Ni, in combination with ionic thermocurrent measurements and Raman spectroscopy. We find that the complex crystal field spectra are due to the coexistence of the Suzuki phase with free impurity-vacancy dipoles. The comparison with the frequencies and the absolute absorption intensities of crystal field transitions in NiCl₂ yield detailed structural information (long range order, point symmetry and anion relaxation) on the Suzuki phase, and an accurate determination of the phase concentrations.