2,500 research outputs found
QuantumATK: An integrated platform of electronic and atomic-scale modelling tools
QuantumATK is an integrated set of atomic-scale modelling tools developed
since 2003 by professional software engineers in collaboration with academic
researchers. While different aspects and individual modules of the platform
have been previously presented, the purpose of this paper is to give a general
overview of the platform. The QuantumATK simulation engines enable
electronic-structure calculations using density functional theory or
tight-binding model Hamiltonians, and also offers bonded or reactive empirical
force fields in many different parametrizations. Density functional theory is
implemented using either a plane-wave basis or expansion of electronic states
in a linear combination of atomic orbitals. The platform includes a long list
of advanced modules, including Green's-function methods for electron transport
simulations and surface calculations, first-principles electron-phonon and
electron-photon couplings, simulation of atomic-scale heat transport, ion
dynamics, spintronics, optical properties of materials, static polarization,
and more. Seamless integration of the different simulation engines into a
common platform allows for easy combination of different simulation methods
into complex workflows. Besides giving a general overview and presenting a
number of implementation details not previously published, we also present four
different application examples. These are calculations of the phonon-limited
mobility of Cu, Ag and Au, electron transport in a gated 2D device, multi-model
simulation of lithium ion drift through a battery cathode in an external
electric field, and electronic-structure calculations of the
composition-dependent band gap of SiGe alloys.Comment: Submitted to Journal of Physics: Condensed Matte
Modeling the iron oxides and oxyhydroxides for the prediction of environmentally sensitive phase transformations
Iron oxides and oxyhydroxides are challenging to model computationally as
competing phases may differ in formation energies by only several kJ/mol, they
undergo magnetization transitions with temperature, their structures may
contain partially occupied sites or long-range ordering of vacancies, and some
loose structures require proper description of weak interactions such as
hydrogen bonding and dispersive forces. If structures and transformations are
to be reliably predicted under different chemical conditions, each of these
challenges must be overcome simultaneously, while preserving a high level of
numerical accuracy and physical sophistication. Here we present comparative
studies of structure, magnetization, and elasticity properties of iron oxides
and oxyhydroxides using density functional theory calculations with plane-wave
and locally-confined-atomic-orbital basis sets, which are implemented in VASP
and SIESTA packages, respectively. We have selected hematite, maghemite,
goethite, lepidocrocite, and magnetite as model systems from a total of 13
known iron oxides and oxyhydroxides; and use same convergence criteria and
almost equivalent settings in order to make consistent comparisons. Our results
show both basis sets can reproduce the energetic stability and magnetic
ordering, and are in agreement with experimental observations. There are
advantages to choosing one basis set over the other, depending on the intended
focus. In our case, we find the method using PW basis set most appropriate, and
combine our results to construct the first phase diagram of iron oxides and
oxyhydroxides in the space of competing chemical potentials, generated entirely
from first principlesComment: 46 pages - Accepted for publication in PRB (19 journal pages),
January 201
Forces and atomic relaxations in the pSIC approach with ultrasoft pseudopotentials
We present the scheme that allows for efficient calculations of forces in the
framework of pseudopotential self-interaction corrected (pSIC) formulation of
the density functional theory. The scheme works with norm conserving and also
with ultrasoft pseudopotentials and has been implemented in the plane-wave
basis code {\sc quantum espresso}. We have performed tests of the internal
consistency of the derived expressions for forces considering ZnO and CeO
crystals. Further, we have performed calculations of equilibrium geometry for
LaTiO, YTiO, and LaMnO perovskites and also for Re and Mn pairs in
silicon. Comparison with standard DFT and DFT+U approaches shows that in the
cases where spurious self-interaction matters, the pSIC approach predicts
different geometry, very often closer to the experimental data.Comment: 11 pages, 2 figure
Functionalized nanopore-embedded electrodes for rapid DNA sequencing
The determination of a patient's DNA sequence can, in principle, reveal an
increased risk to fall ill with particular diseases [1,2] and help to design
"personalized medicine" [3]. Moreover, statistical studies and comparison of
genomes [4] of a large number of individuals are crucial for the analysis of
mutations [5] and hereditary diseases, paving the way to preventive medicine
[6]. DNA sequencing is, however, currently still a vastly time-consuming and
very expensive task [4], consisting of pre-processing steps, the actual
sequencing using the Sanger method, and post-processing in the form of data
analysis [7]. Here we propose a new approach that relies on functionalized
nanopore-embedded electrodes to achieve an unambiguous distinction of the four
nucleic acid bases in the DNA sequencing process. This represents a significant
improvement over previously studied designs [8,9] which cannot reliably
distinguish all four bases of DNA. The transport properties of the setup
investigated by us, employing state-of-the-art density functional theory
together with the non-equilibrium Green's Function method, leads to current
responses that differ by at least one order of magnitude for different bases
and can thus provide a much more robust read-out of the base sequence. The
implementation of our proposed setup could thus lead to a viable protocol for
rapid DNA sequencing with significant consequences for the future of genome
related research in particular and health care in general.Comment: 12 pages, 5 figure
Algorithmic differentiation and the calculation of forces by quantum Monte Carlo
We describe an efficient algorithm to compute forces in quantum Monte Carlo
using adjoint algorithmic differentiation. This allows us to apply the space
warp coordinate transformation in differential form, and compute all the 3M
force components of a system with M atoms with a computational effort
comparable with the one to obtain the total energy. Few examples illustrating
the method for an electronic system containing several water molecules are
presented. With the present technique, the calculation of finite-temperature
thermodynamic properties of materials with quantum Monte Carlo will be feasible
in the near future.Comment: 32 pages, 4 figure, to appear in The Journal of Chemical Physic
A first principles study of wurtzite-structure MnO
We present results of a density functional theory study of MnO in the
wurtzite structure. Our motivation is provided by recent experiments reporting
ferromagnetism in Mn-doped wurtzite structure ZnO. We find that wurtzite MnO a)
is not strongly energetically disfavored as compared with the ground state
rocksalt MnO, b) shows strong magnetostructural coupling and c) has a
piezoelectric response that is larger than that of ZnO. These predictions augur
well for the creation of ferromagnetic piezoelectric semiconductor based on
Mn-doped ZnO
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