185 research outputs found
Energetics of the AK13 Semi-Local Kohn-Sham Exchange Energy Functional
The recent non-empirical semi-local exchange functional of Armiento and
K\"ummel, the AK13 [PRL 111, 036402 (2013)] incorporates a number of features
reproduced by higher-order theory. The AK13 potential behaves analogously with
the discontinuous jump associated with the derivative discontinuity at integer
particle numbers. Recent works have established that AK13 gives a qualitatively
improved orbital description compared to other semi-local methods, and
reproduces a band structure closer to higher-order theory. However, its
energies and energetics are inaccurate. The present work further investigates
the deficiency in energetics. In addition to AK13 results, we find that
applying the local-density approximation (LDA) non-self-consistently on the
converged AK13 density gives very reasonable energetics with equilibrium
lattice constants and bulk moduli well described across 14 systems. We also
confirm that the attractive orbital features of AK13 are retained even after
full structural relaxation. Hence, the deficient energetics cannot be a result
of the AK13 orbitals having adversely affected the quality of the electron
density compared to that of usual semi-local functionals; an improved orbital
description and good energetics are not in opposition. We also prove that the
non-self-consistent scheme is equivalent to using a single external-potential
dependent functional in an otherwise consistent KS-DFT scheme. Furthermore, our
results also demonstrate that, while an internally consistent KS functional is
presently missing, non-self-consistent LDA on AK13 orbitals works as a
practical non-empirical computational scheme to predict geometries, bulk
moduli, while retaining the band structure features of AK13 at the
computational cost of semi-local DFT.Comment: 7 pages, 4 figure
Quantum oscillations in the kinetic energy density: Gradient corrections from the Airy gas
We derive a closed form expression for the quantum corrections to the kinetic
energy density (KED) in the Thomas-Fermi (TF) limit of a linear potential model
system in three dimensions (the Airy gas). The universality of the expression
is tested numerically in a number of three dimensional model systems: (i)
jellium surfaces, (ii) hydrogen-like potentials, (iii) systems confined by an
harmonic potential in one and (iv) all three dimensions, and (v) a system with
a cosine potential (the Mathieu gas). Our results confirm that the usual
gradient expansion of extended Thomas-Fermi theory (ETF) does not describe the
quantum oscillations for systems that incorporate surface regions where the
electron density drops off to zero. We find that the correction derived from
the Airy gas is universally applicable to relevant spatial regions of systems
of type (i), (ii), and (iv), but somewhat surprisingly not (iii). We discuss
possible implications of our findings to the development of functionals for the
kinetic energy density.Comment: 15 pages, 9 figure
On the challenge to improve the density response with unusual gradient approximations
Certain excitations, especially ones of long-range charge transfer character,
are poorly described by time-dependent density functional theory (TDDFT) when
typical (semi-)local functionals are used. A proper description of these
excitations would require an exchange-correlation response differing
substantially from the usual (semi-)local one. It has recently been shown that
functionals of the generalized gradient approximation (GGA) type can yield
unusual potentials, mimicking features of the exact exchange derivative
discontinuity and showing divergences on orbital nodal surfaces. We here
investigate whether these unusual potential properties translate into
beneficial response properties. Using the Sternheimer formalism we closely
investigate the response obtained with the 2013 exchange approximation by
Armiento and K\"ummel (AK13) and the 1988 exchange approximation by Becke
(B88), both of which show divergences on orbital nodal planes. Numerical
calculations for Na2 as well as analytical and numerical calculations for the
hydrogen atom show that the response of AK13 behaves qualitatively different
from usual semi local functionals. However, the AK13 functional leads to
fundamental instabilities in the asymptotic region that prevent its practical
application in TDDFT. Our findings may help the development of future improved
functionals, and corroborate that the frequency-dependent Sternheimer formalism
is excellently suited for running and analyzing TDDFT calculations
A Stimuli-Responsive Nanocomposite for 3D Anisotropic Cell-Guidance and Magnetic Soft Robotics
Stimuli-responsive materials have the potential to enable the generation of new bioinspired devices with unique physicochemical properties and cell-instructive ability. Enhancing biocompatibility while simplifying the production methodologies, as well as enabling the creation of complex constructs, i.e., via 3D (bio)printing technologies, remains key challenge in the field. Here, a novel method is presented to biofabricate cellularized anisotropic hybrid hydrogel through a mild and biocompatible process driven by multiple external stimuli: magnetic field, temperature, and light. A low-intensity magnetic field is used to align mosaic iron oxide nanoparticles (IOPs) into filaments with tunable size within a gelatin methacryloyl matrix. Cells seeded on top or embedded within the hydrogel align to the same axes of the IOPs filaments. Furthermore, in 3D, C2C12 skeletal myoblasts differentiate toward myotubes even in the absence of differentiation media. 3D printing of the nanocomposite hydrogel is achieved and creation of complex heterogeneous structures that respond to magnetic field is demonstrated. By combining the advanced, stimuli-responsive hydrogel with the architectural control provided by bioprinting technologies, 3D constructs can also be created that, although inspired by nature, express functionalities beyond those of native tissue, which have important application in soft robotics, bioactuators, and bionic devices
Optimization of loading protocols for tissue engineering experiments
Tissue engineering (TE) combines cells and biomaterials to treat orthopedic pathologies. Maturation of de novo tissue is highly dependent on local mechanical environments. Mechanical stimulation influences stem cell differentiation, however, the role of various mechanical loads remains unclear. While bioreactors simplify the complexity of the human body, the potential combination of mechanical loads that can be applied make it difficult to assess how different factors interact. Human bone marrow-derived mesenchymal stromal cells were seeded within a fibrin-polyurethane scaffold and exposed to joint-mimicking motion. We applied a full factorial design of experiment to investigate the effect that the interaction between different mechanical loading parameters has on biological markers. Additionally, we employed planned contrasts to analyze differences between loading protocols and a linear mixed model with donor as random effect. Our approach enables screening of multiple mechanical loading combinations and identification of significant interactions that could not have been studied using classical mechanobiology studies. This is useful to screen the effect of various loading protocols and could also be used for TE experiments with small sample sizes and further combinatorial medication studies
High-throughput screening of perovskite alloys for piezoelectric performance and thermodynamic stability
We screen a large chemical space of perovskite alloys for systems with optimal properties to accommodate a morphotropic phase boundary (MPB) in their composition-temperature phase diagram, a crucial feature for high piezoelectric performance. We start from alloy end points previously identified in a high-throughput computational search. An interpolation scheme is used to estimate the relative energies between different perovskite distortions for alloy compositions with a minimum of computational effort. Suggested alloys are further screened for thermodynamic stability. The screening identifies alloy systems already known to host an MPB and suggests a few others that may be promising candidates for future experiments. Our method of investigation may be extended to other perovskite systems, e.g., (oxy-)nitrides, and provides a useful methodology for any application of high-throughput screening of isovalent alloy systems
High-throughput screening of perovskite alloys for piezoelectric performance and thermodynamic stability
We screen a large chemical space of perovskite alloys for systems with optimal properties to accommodate a morphotropic phase boundary (MPB) in their composition-temperature phase diagram, a crucial feature for high piezoelectric performance. We start from alloy end points previously identified in a high-throughput computational search. An interpolation scheme is used to estimate the relative energies between different perovskite distortions for alloy compositions with a minimum of computational effort. Suggested alloys are further screened for thermodynamic stability. The screening identifies alloy systems already known to host an MPB and suggests a few others that may be promising candidates for future experiments. Our method of investigation may be extended to other perovskite systems, e.g., (oxy-)nitrides, and provides a useful methodology for any application of high-throughput screening of isovalent alloy systems
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