15,886 research outputs found
The Small Unit Cell Reconstructions of SrTiO3 (111)
We analyze the basic structural units of simple reconstructions of the (111)
surface of SrTiO3 using density functional calculations. The prime focus is to
answer three questions: what is the most appropriate functional to use; how
accurate are the energies; what are the dominant low-energy structures and
where do they lie on the surface phase diagram. Using test calculations of
representative small molecules we compare conventional GGA with higher-order
methods such as the TPSS meta-GGA and on-site hybrid methods PBE0 and TPSSh,
the later being the most accurate. There are large effects due to reduction of
the metal d oxygen sp hybridization when using the hybrid methods which are
equivalent to a dynamical GGA+U, which leads to rather substantial improvements
in the atomization energies of simple calibration molecules, even though the
d-electron density for titanium compounds is rather small. By comparing the
errors of the different methods we are able to generate an estimate of the
theoretical error, which is about 0.25eV per 1x1 unit cell, with changes of
0.5-1.0 eV per 1x1 cell with the more accurate method relative to conventional
GGA. An analysis of the plausible structures reveals an unusual low-energy
TiO2-rich configuration with an unexpected distorted trigonal biprismatic
structure. This structure can act as a template for layers of either TiO or
Ti2O3, consistent with experimental results as well as, in principle, Magnelli
phases. The results also suggest that both the fracture surface and the
stoichiometric SrTiO3 (111) surface should spontaneously disproportionate into
SrO and TiO2 rich domains, and show that there are still surprises to be found
for polar oxide surfaces.Comment: 14 pages, 4 Figure
Twist-averaged Boundary Conditions in Continuum Quantum Monte Carlo
We develop and test Quantum Monte Carlo algorithms which use a``twist'' or a
phase in the wave function for fermions in periodic boundary conditions. For
metallic systems, averaging over the twist results in faster convergence to the
thermodynamic limit than periodic boundary conditions for properties involving
the kinetic energy with the same computational complexity. We determine
exponents for the rate of convergence to the thermodynamic limit for the
components of the energy of coulomb systems. We show results with twist
averaged variational Monte Carlo on free particles, the Stoner model and the
electron gas using Hartree-Fock, Slater-Jastrow, three-body and backflow
wavefunction. We also discuss the use of twist averaging in the grand canonical
ensemble, and numerical methods to accomplish the twist averaging.Comment: 8 figures, 12 page
Energy Density Functionals From the Strong-Coupling Limit Applied to the Anions of the He Isoelectronic Series
Anions and radicals are important for many applications including
environmental chemistry, semiconductors, and charge transfer, but are poorly
described by the available approximate energy density functionals. Here we test
an approximate exchange-correlation functional based on the exact
strong-coupling limit of the Hohenberg-Kohn functional on the prototypical case
of the He isoelectronic series with varying nuclear charge , which
includes weakly bound negative ions and a quantum phase transition at a
critical value of , representing a big challenge for density functional
theory. We use accurate wavefunction calculations to validate our results,
comparing energies and Kohn-Sham potentials, thus also providing useful
reference data close to and at the quantum phase transition. We show that our
functional is able to bind H and to capture in general the physics of
loosely bound anions, with a tendency to strongly overbind that can be proven
mathematically. We also include corrections based on the uniform electron gas
which improve the results.Comment: Accepted for the JCP Special Topic Issue "Advances in DFT
Methodology
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