24,066 research outputs found
Assessment of two hybrid van der Waals density functionals for covalent and non-covalent binding of molecules
Two hybrid van der Waals density functionals (vdW-DFs) are constructed using
25%, Fock exchange with i) the consistent-exchange vdW-DF-cx functional and ii)
with the vdW-DF2 functional. The ability to describe covalent and non-covalent
binding properties of molecules are assessed. For properties related to
covalent binding, atomization energies (G2-1 set), molecular reaction energies
(G2RC set), as well as ionization energies (G21IP set) are benchmarked against
experimental reference values. We find that hybrid-vdW-DF-cx yields results
that are rather similar to those of the standard non-empirical hybrid PBE0 [JCP
110, 6158 (1996)]. Hybrid vdW-DF2 follows somewhat different trends, showing on
average significantly larger deviations from the reference energies, with a MAD
of 14.5 kcal/mol for the G2-1 set. Non-covalent binding properties of molecules
are assessed using the S22 benchmark set of non-covalently bonded dimers and
the X40 set of dimers of small halogenated molecules, using wavefunction-based
quantum chemistry results for references. For the S22 set, hybrid-vdW-DF-cx
performs better than standard vdW-DF-cx for the mostly hydrogen-bonded systems.
Hybrid-vdW-DF2 offers a slight improvement over standard vdW-DF2. Similar
trends are found for the X40 set, with hybrid-vdW-DF-cx performing particularly
well for binding involving the strongly polar hydrogen halides, but poorly for
systems with tiny binding energies. Our study of the X40 set reveals both the
potential of mixing Fock exchange with vdW-DF, but also highlights shortcomings
of the hybrids constructed here. The solid performance of hybrid-vdW-DF-cx for
covalent-bonded systems, as well as the strengths and issues uncovered for
non-covalently bonded systems, makes this study a good starting point for
developing even more precise hybrid vdW-DFs
van der Waals density functional calculations of binding in molecular crystals
A recent paper [J. Chem. Phys. 132, 134705 (2010)] illustrated the potential
of the van der Waals density functional (vdW-DF) method [Phys. Rev. Lett. 92,
246401 (2004)] for efficient first-principle accounts of structure and cohesion
in molecular crystals. Since then, modifications of the original vdW-DF version
(identified as vdW-DF1) has been proposed, and there is also a new version
called vdW-DF2 [ArXiv 1003.5255], within the vdW-DF framework. Here we
investigate the performance and nature of the modifications and the new version
for the binding of a set of simple molecular crystals: hexamine, dodecahedrane,
C60, and graphite. These extended systems provide benchmarks for computational
methods dealing with sparse matter. We show that a previously documented
enhancement of non-local correlations of vdW-DF1 over an asymptotic atom-based
account close to and a few A, beyond binding separation persists in vdW-DF2.
The calculation and analysis of the binding in molecular crystals requires
appropriate computational tools. In this paper, we also present details on our
real-space parallel implementation of the vdW-DF correlation and on the method
used to generate asymptotic atom-based pair potentials based on vdW-DF.Comment: 5 pages, 4 figure
Van der Waals density-functional theory study for bulk solids with BCC, FCC, and diamond structures
Proper inclusion of van der Waals (vdW) interactions in theoretical
simulations based on standard density functional theory (DFT) is crucial to
describe the physics and chemistry of systems such as organic and layered
materials. Many encouraging approaches have been proposed to combine vdW
interactions with standard approximate DFT calculations. Despite many vdW
studies, there is no consensus on the reliability of vdW methods. To help
further development of vdW methods, we have assessed various vdW functionals
through the calculation of structural prop- erties at equilibrium, such as
lattice constants, bulk moduli, and cohesive energies, for bulk solids,
including alkali, alkali-earth, and transition metals, with BCC, FCC, and
diamond structures as the ground state structure. These results provide
important information for the vdW-related materials research, which is
essential for designing and optimizing materials systems for desired physical
and chemical properties.Comment: 10 pages, 6 Figures, 3 Table
Extent of Fock-exchange mixing for a hybrid van der Waals density functional?
The vdW-DF-cx0 exchange-correlation hybrid design has a truly nonlocal
correlation component and aims to facilitate concurrent descriptions of both
covalent and non-covalent molecular interactions. The vdW-DF-cx0 design mixes a
fixed ratio, , of Fock exchange into the consistent-exchange van der Waals
density functional, vdW-DF-cx. The mixing value is sometimes taken as a
semi-empirical parameter in hybrid formulations. Here, instead, we assert a
plausible optimum average value for the vdW-DF-cx0 design from a formal
analysis; A new, independent determination of the mixing is necessary since
the Becke fit, yielding , is restricted to semilocal correlation and
does not reflect non-covalent interactions. To proceed, we adapt the so-called
two-legged hybrid construction to a starting point in the vdW-DF-cx functional.
For our approach, termed vdW-DF-tlh, we estimate the properties of the
adiabatic-connection specification of the exact exchange-correlation
functional, by combining calculations of the Fock exchange and of the
coupling-constant variation in vdW-DF-cx. We find that such vdW-DF-tlh hybrid
constructions yield accurate characterizations of molecular. The accuracy
motivates trust in the vdW-DF-tlh determination of system-specific values of
the Fock-exchange mixing. We find that an average value best
characterizes the vdW-DF-tlh description of covalent and non-covalent
interactions, although there exists some scatter. This finding suggests that
the original Becke value, , also represents an optimal average
Fock-exchange mixing for the new, truly nonlocal-correlation hybrids. To enable
self-consistent calculations, we furthermore define and test a zero-parameter
hybrid functional vdW-DF-cx0p (having fixed mixing ) and document that
this truly nonlocal correlation hybrid works for general molecular
interactions.Comment: 18 pages, 5 figures, accepted by J. Chem. Phy
Controlled Synthesis of Organic/Inorganic van der Waals Solid for Tunable Light-matter Interactions
Van der Waals (vdW) solids, as a new type of artificial materials that
consist of alternating layers bonded by weak interactions, have shed light on
fascinating optoelectronic device concepts. As a result, a large variety of vdW
devices have been engineered via layer-by-layer stacking of two-dimensional
materials, although shadowed by the difficulties of fabrication. Alternatively,
direct growth of vdW solids has proven as a scalable and swift way, highlighted
by the successful synthesis of graphene/h-BN and transition metal
dichalcogenides (TMDs) vertical heterostructures from controlled vapor
deposition. Here, we realize high-quality organic and inorganic vdW solids,
using methylammonium lead halide (CH3NH3PbI3) as the organic part (organic
perovskite) and 2D inorganic monolayers as counterparts. By stacking on various
2D monolayers, the vdW solids behave dramatically different in light emission.
Our studies demonstrate that h-BN monolayer is a great complement to organic
perovskite for preserving its original optical properties. As a result,
organic/h-BN vdW solid arrays are patterned for red light emitting. This work
paves the way for designing unprecedented vdW solids with great potential for a
wide spectrum of applications in optoelectronics
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