195 research outputs found
First-principles study of structural, elastic, and bonding properties of pyrochlores
Density Functional Theory calculations have been performed to obtain lattice
parameters, elastic constants, and electronic properties of ideal pyrochlores
with the composition ABO (where A=La,Y and B=Ti,Sn,Hf, Zr). Some
thermal properties are also inferred from the elastic properties. A decrease of
the sound velocity (and thus, of the Debye temperature) with the atomic mass of
the B ion is observed. Static and dynamical atomic charges are obtained to
quantify the degree of covalency/ionicity. A large anomalous contribution to
the dynamical charge is observed for Hf, Zr, and specially for Ti. It is
attributed to the hybridization between occupied states of oxygen and
unoccupied d states of the B cation. The analysis based on Mulliken population
and deformation charge integrated in the Voronoi polyhedra indicates that the
ionicity of these pyrochlores increases in the order Sn--Ti--Hf--Zr. The charge
deformation contour plots support this assignment.Comment: Modified contact details, and acknowledgment
Two exchange-correlation functionals compared for first-principles liquid water
The first-principles description of liquid water using ab initio molecular
dynamics (AIMD) based on Density Functional theory (DFT) has recently been
found to require long equilibration times, giving too low diffusivities and a
clear over-structuring of the liquid.
In the light of these findings we compare here the room-temperature
description offered by two different exchange correlation functionals: BLYP,
the most popular for liquid water so far, and RPBE, a revision of the widely
used PBE.
We find for RPBE a less structured liquid with radial distribution functions
closer to the experimental ones than the ones of BLYP.
The diffusivity obtained with RPBE for heavy water is still 20% lower than
the corresponding experimental value, but it represents a substantial
improvement on the BLYP value, one order of magnitude lower than experiment.
These characteristics and the hydrogen-bond (HB) network imperfection point
to an effective temperature ~3% lower than the actual simulation temperature
for the RPBE liquid, as compared with BLYP's ~17% deviation.
The too long O--O average nearest-neighbor distance observed points to an
excessively weak HB, possibly compensating more fundamental errors in the DFT
description.Comment: Jorunal reference adde
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