128 research outputs found
Optimization Algorithm for the Generation of ONCV Pseudopotentials
We present an optimization algorithm to construct pseudopotentials and use it
to generate a set of Optimized Norm-Conserving Vanderbilt (ONCV)
pseudopotentials for elements up to Z=83 (Bi) (excluding Lanthanides). We
introduce a quality function that assesses the agreement of a pseudopotential
calculation with all-electron FLAPW results, and the necessary plane-wave
energy cutoff. This quality function allows us to use a Nelder-Mead
optimization algorithm on a training set of materials to optimize the input
parameters of the pseudopotential construction for most of the periodic table.
We control the accuracy of the resulting pseudopotentials on a test set of
materials independent of the training set. We find that the automatically
constructed pseudopotentials provide a good agreement with the all-electron
results obtained using the FLEUR code with a plane-wave energy cutoff of
approximately 60 Ry.Comment: 11 pages, 6 figure
Soft and transferable pseudopotentials from multi-objective optimization
Ab initio pseudopotentials are a linchpin of modern molecular and condensed
matter electronic structure calculations. In this work, we employ
multi-objective optimization to maximize pseudopotential softness while
maintaining high accuracy and transferability. To accomplish this, we develop a
formulation in which softness and accuracy are simultaneously maximized, with
accuracy determined by the ability to reproduce all-electron energy differences
between Bravais lattice structures, whereupon the resulting Pareto frontier is
scanned for the softest pseudopotential that provides the desired accuracy in
established transferability tests. We employ an evolutionary algorithm to solve
the multi-objective optimization problem and apply it to generate a
comprehensive table of optimized norm-conserving Vanderbilt (ONCV)
pseudopotentials (https://github.com/SPARC-X/SPMS-psps). We show that the
resulting table is softer than existing tables of comparable accuracy, while
more accurate than tables of comparable softness. The potentials thus afford
the possibility to speed up calculations in a broad range of applications areas
while maintaining high accuracy.Comment: 13 pages, 4 figure
Dielectric Screening by 2D Substrates
Two-dimensional (2D) materials are increasingly being used as active
components in nanoscale devices. Many interesting properties of 2D materials
stem from the reduced and highly non-local electronic screening in two
dimensions. While electronic screening within 2D materials has been studied
extensively, the question still remains of how 2D substrates screen charge
perturbations or electronic excitations adjacent to them. Thickness-dependent
dielectric screening properties have recently been studied using electrostatic
force microscopy (EFM) experiments. However, it was suggested that some of the
thickness-dependent trends were due to extrinsic effects. Similarly, Kelvin
probe measurements (KPM) indicate that charge fluctuations are reduced when BN
slabs are placed on SiO, but it is unclear if this effect is due to
intrinsic screening from BN. In this work, we use first principles calculations
to study the fully non-local dielectric screening properties of 2D material
substrates. Our simulations give results in good qualitative agreement with
those from EFM experiments, for hexagonal boron nitride (BN), graphene and
MoS, indicating that the experimentally observed thickness-dependent
screening effects are intrinsic to the 2D materials. We further investigate
explicitly the role of BN in lowering charge potential fluctuations arising
from charge impurities on an underlying SiO substrate, as observed in the
KPM experiments. 2D material substrates can also dramatically change the
HOMO-LUMO gaps of adsorbates, especially for small molecules, such as benzene.
We propose a reliable and very quick method to predict the HOMO-LUMO gap of
small physisorbed molecules on 2D and 3D substrates, using only the band gap of
the substrate and the gas phase gap of the molecule.Comment: 24 pages, 5 figures, Supplementary Informatio
Two-level Chebyshev filter based complementary subspace method: pushing the envelope of large-scale electronic structure calculations
We describe a novel iterative strategy for Kohn-Sham density functional
theory calculations aimed at large systems (> 1000 electrons), applicable to
metals and insulators alike. In lieu of explicit diagonalization of the
Kohn-Sham Hamiltonian on every self-consistent field (SCF) iteration, we employ
a two-level Chebyshev polynomial filter based complementary subspace strategy
to: 1) compute a set of vectors that span the occupied subspace of the
Hamiltonian; 2) reduce subspace diagonalization to just partially occupied
states; and 3) obtain those states in an efficient, scalable manner via an
inner Chebyshev-filter iteration. By reducing the necessary computation to just
partially occupied states, and obtaining these through an inner Chebyshev
iteration, our approach reduces the cost of large metallic calculations
significantly, while eliminating subspace diagonalization for insulating
systems altogether. We describe the implementation of the method within the
framework of the Discontinuous Galerkin (DG) electronic structure method and
show that this results in a computational scheme that can effectively tackle
bulk and nano systems containing tens of thousands of electrons, with chemical
accuracy, within a few minutes or less of wall clock time per SCF iteration on
large-scale computing platforms. We anticipate that our method will be
instrumental in pushing the envelope of large-scale ab initio molecular
dynamics. As a demonstration of this, we simulate a bulk silicon system
containing 8,000 atoms at finite temperature, and obtain an average SCF step
wall time of 51 seconds on 34,560 processors; thus allowing us to carry out 1.0
ps of ab initio molecular dynamics in approximately 28 hours (of wall time).Comment: Resubmitted version (version 2
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