87 research outputs found
A many-body interatomic potential for ionic systems: application to MgO
An analytic representation of the short-range repulsion energy in ionic
systems is described that allows for the fact that ions may change their size
and shape depending on their environment. This function is extremely efficient
to evaluate relative to previous methods of modeling the same physical effects.
Using a well-defined parametrization procedure we have obtained parameter sets
for this energy function that reproduce closely the density functional theory
potential energy surface of bulk MgO. We show how excellent agreement can be
obtained with experimental measurements of phonon frequencies and temperature
and pressure dependences of the density by using this effective potential in
conjunction with ab initio parametrization.Comment: To appear in Journal of Chemical Physics (Oct 15th 2003
How well do Car-Parrinello simulations reproduce the Born-Oppenheimer surface ? Theory and Examples
We derive an analytic expression for the average difference between the
forces on the ions in a Car-Parrinello simulation and the forces obtained at
the same ionic positions when the electrons are at their ground state. We show
that for common values of the fictitious electron mass, a systematic bias may
affect the Car-Parrinello forces in systems where the electron-ion coupling is
large. We show that in the limit where the electronic orbitals are rigidly
dragged by the ions the difference between the two dynamics amounts to a
rescaling of the ionic masses, thereby leaving the thermodynamics intact. We
study the examples of crystalline magnesium oxide and crystalline and molten
silicon. We find that for crystalline silicon the errors are very small. For
crystalline MgO the errors are very large but the dynamics can be quite well
corrected within the rigid-ion model. We conclude that it is important to
control the effect of the electron mass parameter on the quantities extracted
from Car-Parrinello simulations.Comment: Submitted to the Journal of Chemical Physic
A polarizable interatomic force field for TiO parameterized using density functional theory
We report a classical interatomic force field for TiO, which has been
parameterized using density functional theory forces, energies, and stresses in
the rutile crystal structure. The reliability of this new classical potential
is tested by evaluating the structural properties, equation of state, phonon
properties, thermal expansion, and some thermodynamic quantities such as
entropy, free energy, and specific heat under constant volume. The good
agreement of our results with {\em ab initio} calculations and with
experimental data, indicates that our force-field describes the atomic
interactions of TiO in the rutile structure very well. The force field can
also describe the structures of the brookite and anatase crystals with good
accuracy.Comment: Accepted for publication in Phys. Rev. B; Changes from v1 include
multiple minor revisions and a re-write of the description of the force field
in Section II
Design of a low band gap oxide ferroelectric: BiTiO
A strategy for obtaining low band gap oxide ferroelectrics based on charge
imbalance is described and illustrated by first principles studies of the
hypothetical compound BiTiO, which is an alternate stacking of
the ferroelectric BiTiO. We find that this compound is
ferroelectric, similar to BiTiO although with a reduced
polarization. Importantly, calculations of the electronic structure with the
recently developed functional of Tran and Blaha yield a much reduced band gap
of 1.83 eV for this material compared to BiTiO. Therefore,
BiTiO is predicted to be a low band gap ferroelectric material
Charging Induced Emission of Neutral Atoms from NaCl Nanocube Corners
Detachment of neutral cations/anions from solid alkali halides can in
principle be provoked by donating/subtracting electrons to the surface of
alkali halide crystals, but generally constitutes a very endothermic process.
However, the amount of energy required for emission is smaller for atoms
located in less favorable positions, such as surface steps and kinks. For a
corner ion in an alkali halide cube the binding is the weakest, so it should be
easier to remove that atom, once it is neutralized. We carried out first
principles density functional calculations and simulations of neutral and
charged NaCl nanocubes, to establish the energetics of extraction of
neutralized corner ions. Following hole donation (electron removal) we find
that detachment of neutral Cl corner atoms will require a limited energy of
about 0.8 eV. Conversely, following the donation of an excess electron to the
cube, a neutral Na atom is extractable from the corner at the lower cost of
about 0.6 eV. Since the cube electron affinity level (close to that a NaCl(100)
surface state, which we also determine) is estimated to lie about 1.8 eV below
vacuum, the overall energy balance upon donation to the nanocube of a zero
energy electron from vacuum will be exothermic. The atomic and electronic
structure of the NaCl(100) surface, and of the nanocube Na and Cl corner
vacancies are obtained and analyzed as a byproduct.Comment: 16 pages, 2 table, 7 figure
Dipole-quadrupole interactions and the nature of phase III of compressed hydrogen
A new class of strongly infrared active structures is identified for phase
III of compressed molecular H2 by constant-pressure ab initio molecular
dynamics and density-functional perturbation calculations. These are planar
quadrupolar structures obtained as a distortion of low-pressure quadrupolar
phases, after they become unstable at about 150 GPa due to a zone-boundary soft
phonon. The nature of the II-III transition and the origin of the IR activity
are rationalized by means of simple electrostatics, as the onset of a
stabilizing dipole-quadrupole interaction.Comment: 4 pages, 3 figures. To appear in Phys. Rev. Let
Solid molecular hydrogen: The Broken Symmetry Phase
By performing constant-pressure variable-cell ab initio molecular dynamics
simulations we find a quadrupolar orthorhombic structure, of symmetry,
for the broken symmetry phase (phase II) of solid H2 at T=0 and P =110 - 150
GPa. We present results for the equation of state, lattice parameters and
vibronic frequencies, in very good agreement with experimental observations.
Anharmonic quantum corrections to the vibrational frequencies are estimated
using available data on H2 and D2. We assign the observed modes to specific
symmetry representations.Comment: 5 pages (twocolumn), 4 Postscript figures. To appear in Phys. Rev.
Let
A quantum fluid of metallic hydrogen suggested by first-principles calculations
It is generally assumed that solid hydrogen will transform into a metallic
alkali-like crystal at sufficiently high pressure. However, some theoretical
models have also suggested that compressed hydrogen may form an unusual
two-component (protons and electrons) metallic fluid at low temperature, or
possibly even a zero-temperature liquid ground state. The existence of these
new states of matter is conditional on the presence of a maximum in the melting
temperature versus pressure curve (the 'melt line'). Previous measurements of
the hydrogen melt line up to pressures of 44 GPa have led to controversial
conclusions regarding the existence of this maximum. Here we report ab initio
calculations that establish the melt line up to 200 GPa. We predict that subtle
changes in the intermolecular interactions lead to a decline of the melt line
above 90 GPa. The implication is that as solid molecular hydrogen is
compressed, it transforms into a low-temperature quantum fluid before becoming
a monatomic crystal. The emerging low-temperature phase diagram of hydrogen and
its isotopes bears analogies with the familiar phases of 3He and 4He, the only
known zero-temperature liquids, but the long-range Coulombic interactions and
the large component mass ratio present in hydrogen would ensure dramatically
different propertiesComment: See related paper: cond-mat/041040
Determination of the (3x3)-Sn/Ge(111) structure by photoelectron diffraction
At a coverage of about 1/3 monolayer, Sn deposited on Ge(111) below 550 forms
a metastable (sqrt3 x sqrt3)R30 phase. This phase continuously and reversibly
transforms into a (3x3) one, upon cooling below 200 K. The photoemission
spectra of the Sn 4d electrons from the (3x3)-Sn/Ge(111) surface present two
components which are attributed to inequivalent Sn atoms in T4 bonding sites.
This structure has been explored by photoelectron diffraction experiments
performed at the ALOISA beamline of the Elettra storage ring in Trieste
(Italy). The modulation of the intensities of the two Sn components, caused by
the backscattering of the underneath Ge atoms, has been measured as a function
of the emission angle at fixed kinetic energies and viceversa. The bond angle
between Sn and its nearest neighbour atoms in the first Ge layer (Sn-Ge1) has
been measured by taking polar scans along the main symmetry directions and it
was found almost equivalent for the two components. The corresponding bond
lengths are also quite similar, as obtained by studying the dependence on the
photoelectron kinetic energy, while keeping the photon polarization and the
collection direction parallel to the Sn-Ge1 bond orientation (bond emission). A
clear difference between the two bonding sites is observed when studying the
energy dependence at normal emission, where the sensitivity to the Sn height
above the Ge atom in the second layer is enhanced. This vertical distance is
found to be 0.3 Angstroms larger for one Sn atom out of the three contained in
the lattice unit cell. The (3x3)-Sn/Ge(111) is thus characterized by a
structure where the Sn atom and its three nearest neighbour Ge atoms form a
rather rigid unit that presents a strong vertical distortion with respect to
the underneath atom of the second Ge layer.Comment: 10 pages with 9 figures, added reference
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