44 research outputs found
Nonlocal Kinetic Energy Functionals By Functional Integration
Since the seminal works of Thomas and Fermi, researchers in the
Density-Functional Theory (DFT) community are searching for accurate electron
density functionals. Arguably, the toughest functional to approximate is the
noninteracting Kinetic Energy, , the subject of this work. The
typical paradigm is to first approximate the energy functional, and then take
its functional derivative, , yielding
a potential that can be used in orbital-free DFT, or subsystem DFT simulations.
Here, this paradigm is challenged by constructing the potential from the
second-functional derivative via functional integration. A new nonlocal
functional for is prescribed (which we dub MGP) having a density
independent kernel. MGP is constructed to satisfy three exact conditions: (1) a
nonzero "Kinetic electron" arising from a nonzero exchange hole; (2) the second
functional derivative must reduce to the inverse Lindhard function in the limit
of homogenous densities; (3) the potential derives from functional integration
of the second functional derivative. Pilot calculations show that MGP is
capable of reproducing accurate equilibrium volumes, bulk moduli, total energy,
and electron densities for metallic (BCC, FCC) and semiconducting (CD) phases
of Silicon as well as of III-V semiconductors. MGP functional is found to be
numerically stable typically reaching selfconsistency within 12 iteration of a
truncated Newton minimization algorithm. MGP's computational cost and memory
requirements are low and comparable to the Wang-Teter (WT) nonlocal functional
or any GGA functional
Constructing optimal local pseudopotentials from first principles
Local pseudopotential (LPP) is an important component of the orbital free
density functional theory (OF-DFT), which is a promising large scale simulation
method that can still maintain information of electron state in materials. Up
to date, LPP is usually extracted from the solid state DFT calculations. It is
unclear how to assess its transferability while applying to a much different
chemical environment. Here we reveal a fundamental relation between the first
principles norm-conserving PP (NCPP) and the LPP. Using the optimized effective
potential method developed for exchange functional, we demonstrate that the LPP
can be constructed optimally from the NCPP for a large number of elements. Our
theory also reveals that the existence of an LPP is intrinsic to the elements,
irrespective to the parameters used for the construction. Our method provides a
unified method in constructing and assessing LPP in the framework of first
principles pseudopotentials
Nonlocal Subsystem Density Functional Theory
By invoking a divide-and-conquer strategy, subsystem DFT dramatically reduces
the computational cost of large-scale, \textit{ab-initio} electronic structure
simulations of molecules and materials. The central ingredient setting
subsystem DFT apart from Kohn-Sham DFT is the non-additive kinetic energy
functional (NAKE). Currently employed NAKEs are at most semilocal (i.e., they
only depend on the electron density and its gradient), and as a result of this
approximation, so far only systems composed of weakly interacting subsystems
have been successfully tackled. In this work, we advance the state-of-the-art
by introducing fully nonlocal NAKEs in subsystem DFT simulations for the first
time. A benchmark analysis based on the S22-5 test set shows that nonlocal
NAKEs considerably improve the computed interaction energies and electron
density compared to commonly employed GGA NAKEs, especially when the
inter-subsystem electron density overlap is high. Most importantly, we resolve
the long standing problem of too attractive interaction energy curves typically
resulting from the use of GGA NAKEs
Orbital-Free DFT Correctly Models Quantum Dots When Asymptotics, Nonlocality and Nonhomogeneity Are Accounted For
Million-atom quantum simulations are in principle feasible with Orbital-Free
Density Functional Theory (OF-DFT) because the algorithms only require simple
functional minimizations with respect to the electron density function. In this
context, OF-DFT has been useful for simulations of warm dense matter, plasma,
cold metals and alloys. Unfortunately, systems as important as quantum dots and
clusters (having highly inhomogeneous electron densities) still fall outside
OF-DFT's range of applicability. In this work, we address this century old
problem by devising and implementing an accurate, transferable and universal
family of nonlocal Kinetic Energy density functionals that feature correct
asymptotics and can handle highly inhomogenous electron densities. For the
first time to date, we show that OF-DFT achieves close to chemical accuracy for
the electronic energy and reproduces the electron density to about 5\% of the
benchmark for semiconductor quantum dots and metal clusters. Therefore, this
work demonstrates that OF-DFT is no longer limited to simulations of systems
with nearly homogeneous electron density but it can venture into simulations of
clusters and quantum dots with applicability to rational design of novel
materials
Absence of BLM leads to accumulation of chromosomal DNA breaks during both unperturbed and disrupted S phases
Bloom's syndrome (BS), a disorder associated with genomic instability and cancer predisposition, results from defects in the Bloom's helicase (BLM) protein. In BS cells, chromosomal abnormalities such as sister chromatid exchanges occur at highly elevated rates. Using Xenopus egg extracts, we have studied Xenopus BLM (Xblm) during both unperturbed and disrupted DNA replication cycles. Xblm binds to replicating chromatin and becomes highly phosphorylated in the presence of DNA replication blocks. This phosphorylation depends on Xenopus ATR (Xatr) and Xenopus Rad17 (Xrad17), but not Claspin. Xblm and Xenopus topoisomerase III{alpha} (Xtop3{alpha}) interact in a regulated manner and associate with replicating chromatin interdependently. Immunodepletion of Xblm from egg extracts results in accumulation of chromosomal DNA breaks during both normal and perturbed DNA replication cycles. Disruption of the interaction between Xblm and Xtop3{alpha} has similar effects. The occurrence of DNA damage in the absence of Xblm, even without any exogenous insult to the DNA, may help to explain the genesis of chromosomal defects in BS cells
A linear scaling method to evaluate the ion-electron potential of crystalline solids
We propose a simple linear scaling expression in reciprocal space for
evaluating the ion--electron potential of crystalline solids. The expression
replaces the long-range ion--electron potential with an equivalent localized
charge distribution and corresponding boundary conditions on the unit cell.
Given that no quadratic scaling structure factor is required---as used in
traditional methods---the expression shows inherent linear behavior, and is
well suited to simulating large-scale systems within orbital-free density
functional theory. The scheme is implemented in the ATLAS software package and
benchmarked by using a solid Mg bcc lattice containing tens of thousands of
atoms in the unit cell. The test results show that the method can efficiently
model large crystals with high computational accuracy.Comment: 5 pages, 5 figure
Ab-initio structure and dynamics of supercritical CO2
Green technologies rely on green solvents and fluids. Among them,
supercritical CO2 already finds many important applications. The molecular
level understanding of the dynamics and structure of this supercritical fluid
is a prerequisite to rational design of future green technologies.
Unfortunately, the commonly employed Kohn-Sham DFT is too computationally
demanding to produce meaningfully converged dynamics within a reasonable time
and with a reasonable computational effort. Thanks to subsystem DFT, we analyze
finite-size effects by considering simulations cells of varying sizes (up to
256 independent molecules in the cell) and finite-time effects by running 100
ps-long trajectories. We find that the simulations are in reasonable and
semiquantitative agreement with the available neutron diffraction experiments
and that, as opposed to the gas phase, the CO2 molecules in the fluid are bent
with an average OCO angle of 175.8 degrees. Our simulations also confirm that
the dimer T-shape is the most prevalent configuration. Our results further
strengthen the experiment-simulation agreement for this fluid when comparing
radial distribution functions and diffusion coefficient, confirming subsystem
DFT as a viable tool for modeling structure and dynamics of condensed-phase
systems
eQE 2.0: Subsystem DFT Beyond GGA Functionals
By adopting a divide-and-conquer strategy, subsystem-DFT (sDFT) can
dramatically reduce the computational cost of large-scale electronic structure
calculations. The key ingredients of sDFT are the nonadditive kinetic energy
and exchange-correlation functionals which dominate it's accuracy. Even though,
semilocal nonadditive functionals find a broad range of applications, their
accuracy is somewhat limited especially for those systems where achieving
balance between exchange-correlation interactions on one side and nonadditive
kinetic energy on the other is crucial. In eQE 2.0, we improve dramatically the
accuracy of sDFT simulations by (1) implementing nonlocal nonadditive kinetic
energy functionals based on the LMGP family of functionals; (2) adapting
Quantum ESPRESSO's implementation of rVV10 and vdW-DF nonlocal
exchange-correlation functionals to be employed in sDFT simulations; (3)
implementing "deorbitalized" meta GGA functionals (e.g., SCAN-L). We carefully
assess the performance of the newly implemented tools on the S22-5 test set.
eQE 2.0 delivers excellent interaction energies compared to conventional
Kohn-Sham DFT and CCSD(T). The improved performance does not come at a loss of
computational efficiency. We show that eQE 2.0 with nonlocal nonadditive
functionals retains the same linear scaling behavior achieved in eQE 1.0 with
semilocal nonadditive functionals
DFTpy: An efficient and object-oriented platform for orbital-free DFT simulations
In silico materials design is hampered by the computational complexity of
Kohn-Sham DFT, which scales cubically with the system size. Owing to the
development of new-generation kinetic energy density functionals (KEDFs),
orbital-free DFT (OFDFT, a linear-scaling method) can now be successfully
applied to a large class of semiconductors and such finite systems as quantum
dots and metal clusters. In this work, we present DFTpy, an open source
software implementing OFDFT written entirely in Python 3 and outsourcing the
computationally expensive operations to third-party modules, such as NumPy and
SciPy. When fast simulations are in order, DFTpy exploits the fast Fourier
transforms (FFTs) from PyFFTW. New-generation, nonlocal and
density-dependent-kernel KEDFs are made computationally efficient by employing
linear splines and other methods for fast kernel builds. We showcase DFTpy by
solving for the electronic structure of a million-atom system of aluminum metal
which was computed on a single CPU. The Python 3 implementation is
object-oriented, opening the door to easy implementation of new features. As an
example, we present a time-dependent OFDFT implementation (hydrodynamic DFT)
which we use to compute the spectra of small metal cluster recovering
qualitatively the time-dependent Kohn-Sham DFT result. The Python code base
allows for easy implementation of APIs. We showcase the combination of DFTpy
and ASE for molecular dynamics simulations (NVT) of liquid metals. DFTpy is
released under the MIT license
ATLAS: A Real-Space Finite-Difference Implementation of Orbital-Free Density Functional Theory
Orbital-free density functional theory (OF-DFT) is a promising method for
large-scale quantum mechanics simulation as it provides a good balance of
accuracy and computational cost. Its applicability to large-scale simulations
has been aided by progress in constructing kinetic energy functionals and local
pseudopotentials. However, the widespread adoption of OF-DFT requires further
improvement in its efficiency and robustly implemented software. Here we
develop a real-space finite-difference method for the numerical solution of
OF-DFT in periodic systems. Instead of the traditional self-consistent method,
a powerful scheme for energy minimization is introduced to solve the
Euler--Lagrange equation. Our approach engages both the real-space
finite-difference method and a direct energy-minimization scheme for the OF-DFT
calculations. The method is coded into the ATLAS software package and
benchmarked using periodic systems of solid Mg, Al, and AlMg. The test
results show that our implementation can achieve high accuracy, efficiency, and
numerical stability for large-scale simulations