676 research outputs found
LAPW vs. LMTO full-potential simulations and anharmonic dynamics of KNbO3
With the aim to get an insight in the origin of differences in the earlier
reported calculation results for KNbO3 and to test the recently proposed
implementation of the FP-LMTO method by Methfessel and van Schilfgaarde, we
perform a comparative study of the ferroelectric instability in KNbO3 by
FP-LMTO and LAPW methods. It is shown that a high precision in the description
of the charge density variations over the interstitial region in perovskite
materials is essential; the technical limitations of the accuracy of
charge-density description apparently accounted for previously reported slight
disagreement with the LAPW results. With more accurate description of the
charge density by sufficiently fine real-space grid, the results obtained by
both methods became almost identical.
In order to extract additional information (beyond the harmonic
approximation) from the total energy fit obtainable in total-energy
calculations, a scheme is proposed to solve the multidimensional vibrational
Schroedinger equation in the model of non-interacting anharmonic oscillators
via the expansion in hyperspherical harmonics.Comment: 11 pages, 2 figures, uses aipproc.sty. Presented at the Fifth
Williamsburg Workshop on First-Principles Calculations for Ferroelectric
Ab initio Simulations of Fe-based Ferric Wheels
Based on first-principles density-functional theory calculations we
investigate the electronic structure of hexanuclear "ferric wheels" M
Fe_6[N(CH_2 CH_2 O)_3]_6 Cl (M = Li, Na) in their antiferromagnetic ground
state. The electronic structure is presented in form of spin- and site-resolved
local densities of states. The latter clearly indicate that the magnetic moment
is distributed over several sites. The local moment at the iron site is still
the largest one with about 4 mu_B, thus indicating the valence state of iron to
be closer to Fe(II) than to commonly accepted Fe(III). The local spin of S=5/2
per iron site, following from magnetization measurements, is perfectly
reproduced if one takes the moments on the neighbor atoms into account. The
largest magnetic polarization is found on the apical oxygen atom, followed by
nitrogen bridging oxygens. These findings are confirmed by a map of spatial
spin density. A further goal of the present study has been a comparative test
of two different DFT implementations, Siesta and NRLMOL. They yield a very good
agreement down to small details in the electronic structure.Comment: 10 pages, 3 embedded postscript figures, to be published in Molecular
Physics Reports (proceedings of the Summer School on New Magnetics - Bedlewo,
Poland, September 2003). Two references update
Limits of structure stability of simple liquids revealed by study of relative fluctuations
We analyse the inverse reduced fluctuations (inverse ratio of relative volume
fluctuation to its value in the hypothetical case where the substance acts an
ideal gas for the same temperature-volume parameters) for simple liquids from
experimental acoustic and thermophysical data along a coexistence line for both
liquid and vapour phases. It has been determined that this quantity has a
universal exponential character within the region close to the melting point.
This behaviour satisfies the predictions of the mean-field (grand canonical
ensemble) lattice fluid model and relates to the constant average structure of
a fluid, i.e. redistribution of the free volume complementary to a number of
vapour particles. The interconnection between experiment-based fluctuational
parameters and self-diffusion characteristics is discussed. These results may
suggest experimental methods for determination of self-diffusion and structural
properties of real substances.Comment: 5 pages, 4 figure
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