30 research outputs found
Temperature dependent effective potential method for accurate free energy calculations of solids
We have developed a thorough and accurate method of determining anharmonic
free energies, the temperature dependent effective potential technique (TDEP).
It is based on \emph{ab initio} molecular dynamics followed by a mapping onto a
model Hamiltonian that describes the lattice dynamics. The formalism and the
numerical aspects of the technique are described in details. A number of
practical examples are given, and results are presented, which confirm the
usefulness of TDEP within \emph{ab initio} and classical molecular dynamics
frameworks. In particular, we examine from first-principles the behavior of
force constants upon the dynamical stabilization of body centered phase of Zr,
and show that they become more localized. We also calculate phase diagram for
He modeled with the Aziz \emph{et al.} potential and obtain results which
are in favorable agreement both with respect to experiment and established
techniques
Finite temperature elastic constants of paramagnetic materials within the disordered local moment picture from ab initio molecular dynamics calculations
We present a theoretical scheme to calculate the elastic constants of
magnetic materials in the high-temperature paramagnetic state. Our approach is
based on a combination of disordered local moments picture and ab initio
molecular dynamics (DLM-MD). Moreover, we investigate a possibility to enhance
the efficiency of the simulations of elastic properties using recently
introduced method: symmetry imposed force constant temperature dependent
effective potential (SIFC-TDEP). We have chosen cubic paramagnetic CrN as a
model system. This is done due to its technological importance and its
demonstrated strong coupling between magnetic and lattice degrees of freedom.
We have studied the temperature dependent single-crystal and polycrystalline
elastic constants of paramagentic CrN up to 1200 K. The obtained results at T=
300 K agree well with the experimental values of polycrystalline elastic
constants as well as Poisson ratio at room temperature. We observe that the
Young's modulus is strongly dependent on temperature, decreasing by ~14% from
T=300 K to 1200 K. In addition we have studied the elastic anisotropy of CrN as
a function of temperature and we observe that CrN becomes substantially more
isotropic as the temperature increases. We demonstrate that the use of Birch
law may lead to substantial errors for calculations of temperature induced
changes of elastic moduli. The proposed methodology can be used for accurate
predictions of mechanical properties of magnetic materials at temperatures
above their magnetic order-disorder phase transition.Comment: 1 table, 3 figure
A Test of Sovereignty: Franchise Tax Board of the State of California v. Gilbert P. Hyatt
In Franchise Tax Board of California v. Hyatt, the Supreme Court considers whether to overrule Nevada v. Hall, a 1979 Supreme Court decision. Hall permitted a State to be haled into the court of another State without its consent. In 2016, an evenly divided Supreme Court affirmed Hall 4-4 when faced with the same question, and following a remand to the Nevada Supreme Court, the Court has granted certiorari on this question once again. This Commentary contends that Hall was wrongly decided and should be overruled. The Constitution’s ratification did not alter the status of common-law State sovereign immunity, leaving intact not only State sovereign immunity in a State’s own court but also a State’s immunity to suits in the courts of another State without consent. However, this case, in which the Petitioner has already appeared in the court of another State, is not the appropriate vehicle for overruling Hall. State sovereign immunity should be restored at the next possible opportunity, when a State properly asks a federal court to enforce its common-law immunity from the courts of a sister State. Sovereigns should enjoy immunity not only in their own courts, but also in the courts of their peers
Wavefunction extended Lagrangian Born-Oppenheimer molecular dynamics
Extended Lagrangian Born-Oppenheimer molecular dynamics [Niklasson, Phys.
Rev. Lett. 100 123004 (2008)] has been generalized to the propagation of the
electronic wavefunctions. The technique allows highly efficient first
principles molecular dynamics simulations using plane wave pseudopotential
electronic structure methods that are stable and energy conserving also under
incomplete and approximate self-consistency convergence. An implementation of
the method within the planewave basis set is presented and the accuracy and
efficiency is demonstrated both for semi-conductor and metallic materials.Comment: 6 pages, 3 figure
Inviwo -- A Visualization System with Usage Abstraction Levels
The complexity of today's visualization applications demands specific
visualization systems tailored for the development of these applications.
Frequently, such systems utilize levels of abstraction to improve the
application development process, for instance by providing a data flow network
editor. Unfortunately, these abstractions result in several issues, which need
to be circumvented through an abstraction-centered system design. Often, a high
level of abstraction hides low level details, which makes it difficult to
directly access the underlying computing platform, which would be important to
achieve an optimal performance. Therefore, we propose a layer structure
developed for modern and sustainable visualization systems allowing developers
to interact with all contained abstraction levels. We refer to this interaction
capabilities as usage abstraction levels, since we target application
developers with various levels of experience. We formulate the requirements for
such a system, derive the desired architecture, and present how the concepts
have been exemplary realized within the Inviwo visualization system.
Furthermore, we address several specific challenges that arise during the
realization of such a layered architecture, such as communication between
different computing platforms, performance centered encapsulation, as well as
layer-independent development by supporting cross layer documentation and
debugging capabilities
Equation of state of paramagnetic CrN from ab initio molecular dynamics
Equation of state for chromium nitride has been debated in the literature in
connection with a proposed collapse of its bulk modulus following the pressure
induced transition from the paramagnetic cubic phase to the antiferromagnetic
orthorhombic phase [F. Rivadulla et al., Nat Mater 8, 974 (2009); B. Alling et
al., Nat Mater 9, 283 (2010)]. Experimentally the measurements are complicated
due to the low transition pressure, while theoretically the simulation of
magnetic disorder represent a major challenge. Here a first-principles method
is suggested for the calculation of thermodynamic properties of magnetic
materials in their high temperature paramagnetic phase. It is based on
ab-initio molecular dynamics and simultaneous redistributions of the disordered
but finite local magnetic moments. We apply this disordered local moments
molecular dynamics method to the case of CrN and simulate its equation of
state. In particular the debated bulk modulus is calculated in the paramagnetic
cubic phase and is shown to be very similar to that of the antiferromagnetic
orthorhombic CrN phase for all considered temperatures.Comment: 7 pages, 4 figure
Extended Lagrangian free energy molecular dynamics
Extended free energy Lagrangians are proposed for first principles molecular
dynamics simulations at finite electronic temperatures for plane-wave
pseudopotential and local orbital density matrix based calculations. Thanks to
the extended Lagrangian description the electronic degrees of freedom can be
integrated by stable geometric schemes that conserve the free energy. For the
local orbital representations both the nuclear and electronic forces have
simple and numerically efficient expressions that are well suited for reduced
complexity calculations. A rapidly converging recursive Fermi operator
expansion method that does not require the calculation of eigenvalues and
eigenfunctions for the construction of the fractionally occupied density matrix
is discussed. An efficient expression for the Pulay force that is valid also
for density matrices with fractional occupation occurring at finite electronic
temperatures is also demonstrated
Vibrational free energy and phase stability of paramagnetic and antiferromagnetic CrN from ab initio
Краткое обоснование и основы организации гидрохимического мониторинга разработки месторождения Западная Курна-2 (Ирак)
Тез. докл. XI Междунар. науч.-техн. конф. (науч. чтения, посвящ. П. О. Сухому), Гомель, 20-21 октября 2016 г