106 research outputs found
A finite element approach to self-consistent field theory calculations of multiblock polymers
Self-consistent field theory (SCFT) has proven to be a powerful tool for
modeling equilibrium microstructures of soft materials, particularly for
multiblock polymers. A very successful approach to numerically solving the SCFT
set of equations is based on using a spectral approach. While widely
successful, this approach has limitations especially in the context of current
technologically relevant applications. These limitations include non-trivial
approaches for modeling complex geometries, difficulties in extending to
non-periodic domains, as well as non-trivial extensions for spatial adaptivity.
As a viable alternative to spectral schemes, we develop a finite element
formulation of the SCFT paradigm for calculating equilibrium polymer
morphologies. We discuss the formulation and address implementation challenges
that ensure accuracy and efficiency. We explore higher order chain contour
steppers that are efficiently implemented with Richardson Extrapolation. This
approach is highly scalable and suitable for systems with arbitrary shapes. We
show spatial and temporal convergence and illustrate scaling on up to 2048
cores. Finally, we illustrate confinement effects for selected complex
geometries. This has implications for materials design for nanoscale
applications where dimensions are such that equilibrium morphologies
dramatically differ from the bulk phases
The importance of the electronic contribution to linear magnetoelectricity
We demonstrate that the electronic contribution to the linear magnetoelectric
response, usually omitted in first-principles studies, can be comparable in
magnitude to that mediated by lattice distortions, even for materials in which
responses are strong. Using a self-consistent Zeeman response to an applied
magnetic field for noncollinear electron spins, we show how electric
polarization emerges in linear magnetoelectrics through both electronic- and
lattice-mediated components -- in analogy with the high- and low-frequency
dielectric response to an electric field. The approach we use is conceptually
and computationally simple, and can be applied to study both linear and
non-linear responses to magnetic fields.Comment: 5 pages, 3 figure
Quantum Monte Carlo Simulation of the High-Pressure Molecular-Atomic Crossover in Fluid Hydrogen
A first-order liquid-liquid phase transition in high-pressure hydrogen
between molecular and atomic fluid phases has been predicted in computer
simulations using ab initio molecular dynamics approaches. However, experiments
indicate that molecular dissociation may occur through a continuous crossover
rather than a first-order transition. Here we study the nature of molecular
dissociation in fluid hydrogen using an alternative simulation technique in
which electronic correlation is computed within quantum Monte Carlo, the
so-called Coupled Electron Ion Monte Carlo (CEIMC) method. We find no evidence
for a first-order liquid-liquid phase transition.Comment: 4 pages, 5 figures; content changed; accepted for publication in
Phys. Rev. Let
Vertex corrections in localized and extended systems
Within many-body perturbation theory we apply vertex corrections to various
closed-shell atoms and to jellium, using a local approximation for the vertex
consistent with starting the many-body perturbation theory from a DFT-LDA
Green's function. The vertex appears in two places -- in the screened Coulomb
interaction, W, and in the self-energy, \Sigma -- and we obtain a systematic
discrimination of these two effects by turning the vertex in \Sigma on and off.
We also make comparisons to standard GW results within the usual random-phase
approximation (RPA), which omits the vertex from both. When a vertex is
included for closed-shell atoms, both ground-state and excited-state properties
demonstrate only limited improvements over standard GW. For jellium we observe
marked improvement in the quasiparticle band width when the vertex is included
only in W, whereas turning on the vertex in \Sigma leads to an unphysical
quasiparticle dispersion and work function. A simple analysis suggests why
implementation of the vertex only in W is a valid way to improve quasiparticle
energy calculations, while the vertex in \Sigma is unphysical, and points the
way to development of improved vertices for ab initio electronic structure
calculations.Comment: 8 Pages, 6 Figures. Updated with quasiparticle neon results, extended
conclusions and references section. Minor changes: Updated references, minor
improvement
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