1,721,495 research outputs found
PRESSURE-INDUCED STRUCTURAL INSTABILITY OF CESIUM-HALIDES FROM ABINITIO PSEUDOPOTENTIAL TECHNIQUES
No full textThe structural stability of the cubic phase of CsI versus tetragonal distortions is studied from first principles using state-of-the-art local-density techniques, namely, norm-conserving pseudopotentials and large plane-wave basis sets. The effects of the polarization of the cation are explicitly accounted for using a pseudopotential that sustains the Cs 5s and 5p bands. We find that, in agreement with recent x-ray diffraction experiments and with previous theoretical work, the tetragonal phase is more stable at volumes smaller than 0.54 of the zero-pressure value. The mechanism of transition is revealed in terms of the balance between the Madelung and repulsive interionic interactions. We find that at volumes smaller than the transition volume the cubic phase is metastable, thus indicating that the transition is first order. The electron-charge-density rearrangements following compression and distortion are also examined. No evidence of a further transition to an even lower-symmetry structure has been found in a preliminary search
The structure and phase stability of CO adsorbates on Rh(110)
No full textThe structure of CO adsorbates on the Rh(110) surface is studied at full coverage using first-principles techniques. The relative energies of different adsorbate geometries are determined by means of accurate structure optimizations. In agreement with experiments. we find that a p2mg(2 x 1)-2CO structure is the most stable. The CO molecules sit on the short-bridge site (carbon below) with the molecular asis slightly tilted off the surface normal, along the (001) direction. Configurations corresponding iv different distributions of tilt angles are mapped onto an anisotropic two-dimensional Ising model whose parameters are extracted from our ab initio calculations. We find that an order-disorder phase-transition occurs at a temperature T-c approximate to 300 K. (C) 1997 Elsevier Science B.V
Exact‐exchange extension of the local‐spin‐density approximation in atoms. II. The iron series
No full texts‐d interconfigurational energies, s‐spin flip energies, and ionization potentials for atoms in the first transition series are calculated within a local‐spin‐density scheme, where the exchange is treated exactly. The results so obtained are in better agreement with experiments than those obtained by the Hartree–Fock (HF) or local‐spin‐density approximations (LSDA), while they are of the same quality as those obtained by the self‐interaction‐corrected (SIC) version of the LSDA. The merits of the proposed scheme with respect to the other mentioned approximations are discussed in detail
Reptation quantum Monte Carlo: A method for unbiased ground-state averages and imaginary-time correlations
No full textWe introduce a new stochastic method for calculating ground-state properties of quantum systems. Segments of a Langevin random walk guided by a trial wave function are subject to a Metropolis rejection test performed on the time integral of the local energy. The algorithm-which is as simple as variational Monte Carlo-for bosons provides exact expectation values of local observables, as well as their static and dynamic (in imaginary time) response functions, without mixed-estimate nor population-control biases. Our method is demonstrated with a few case applications to (4)He
Dependence of the crystal lattice constant on isotopic composition: Theory and ab initio calculations for C, Si, and Ge
No full textWe consider the problem of the dependence of the lattice constant of a crystal upon its isotopic purity. After giving a brief account of the theoretical framework, we report on first-principles calculations for diamond, silicon, and germanium, performed by treating nuclear vibrations by density-functional perturbation theory. Our results are in good agreement with existing experimental data for diamond and germanium, and provide reliable predictions for silicon, which are relevant for metrological applications
Ab initio calculation of the macroscopic dielectric constant in silicon
No full textWe perform a first-principles calculation of the static dielectric constant of Si in the framework of density-functional theory. The only essential approximation used in this work is the local-density approximation (LDA): norm-conserving pseudopotentials and large plane-wave basis sets are used, numerical roundoff and convergence errors are kept below 1%. The present calculation gives for the first time the ‘‘exact’’ value of the macroscopic dielectric constant at the LDA level. The theoretical value of ε∞ is 12% higher than experiment
On the Green's function technique for the study of multiphoton transitions in atoms
No full textThe Green's function method for implicitly evaluating the sums over intermediate states occurring in the study of multiphoton processes is revisited. Partieular emphasis is given to bound-bound transitions and to the numerical techniques used to implement the method. This is also extended to the important case in which the sum over intermediate states must be performed with respect to an incomplete set. The accuracy of the method is tested against very precise results on atomic hydrogen obtained by direct summation over intermediate states
Beyond the local-spin-density approximation in atoms: exact-exchange extension of the theory
No full textEffects of isotopic disorder on the Raman spectra of crystals: theory and ab initio calculations for diamond and germanium
No full textWe present a method to study the effects of isotopic composition on the Raman spectra of crystals, in which disorder is treated exactly without resorting to any kind of mean-field approximation. The Raman cross section is expressed in terms of a suitable diagonal element of the vibrational Green’s function, which is accurately and efficiently calculated using the recursion technique. This method can be used in conjunction with both semiempirical lattice-dynamical models and with first-principles interatomic force constants. We have applied our technique to diamond and germanium using the most accurate interatomic force constants presently available, obtained from density-functional perturbation theory. Our method correctly reproduces the light scattering in diamond—where isotopic effects dominates over the anharmonic ones—as well as in germanium, where anharmonic effects are larger
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