9 research outputs found
Bending Two-Dimensional Materials To Control Charge Localization and Fermi-Level Shift
High-performance
electronics requires the fine control of semiconductor conductivity.
In atomically thin two-dimensional (2D) materials, traditional doping
technique for controlling carrier concentration and carrier type may
cause crystal damage and significant mobility reduction. Contact engineering
for tuning carrier injection and extraction and carrier type may suffer
from strong Fermi-level pinning. Here, using first-principles calculations,
we predict that mechanical bending, as a unique attribute of thin
2D materials, can be used to control conductivity and Fermi-level
shift. We find that bending can control the charge localization of
top valence bands in both MoS<sub>2</sub> and phosphorene nanoribbons.
The donor-like in-gap edge-states of armchair MoS<sub>2</sub> ribbon
and their associated Fermi-level pinning can be removed by bending.
A bending-controllable new in-gap state and accompanying directâindirect
gap transition are predicted in armchair phosphorene nanoribbon. We
demonstrate that such emergent bending effects are realizable. The
bending stiffness as well as the effective thickness of 2D materials
are also derived from first principles. Our results are of fundamental
and technological relevance and open new routes for designing functional
2D materials for applications in which flexuosity is essential
Density Functional Theory Study of Controllable Optical Absorptions and Magneto-Optical Properties of Magnetic CrI<sub>3</sub> Nanoribbons: Implications for Compact 2D Magnetic Devices
A chromium triiodide (CrI3) monolayer has
an interesting
ferromagnetic ground state. In this work, we calculate band structures
and magnetic moments of tensile-strained and bent zigzag CrI3 nanoribbons with density functional theory. The edge iodine atoms
form flat low-lying conduction bands and couple with chromium atoms
ferromagnetically, while the non-edge iodine atoms weakly couple antiferromagnetically.
Narrow CrI3 nanoribbons have two locally stable magnetic
moment orientations, namely, out-of-plane and in-plane (along the
nanoribbon periodic direction) configurations. This enables four magnetization
states in CrI3 nanoribbons, including two out-of-plane
ones (up and down) and two in-plane ones (forward and backward along
the nanoribbon periodical direction), increasing the operating controllability.
Based on the one-dimensional Ising spin chain model, the spin correlation
length of the narrow CrI3 nanoribbon is estimated to be
about 10 Ă
at its estimated Curie temperature of 27 K, which
is lower than the measured 45 K of the monolayer CrI3.
The optical absorption and magneto-optical properties of CrI3 nanoribbons are investigated with many-body perturbation GW-BSE
(BetheâSalpeter equation), including magnetic dichroism and
Faraday and magneto-optical Kerr effects. The low-energy dark excitons
are mainly from transitions between electrons and holes with unlike
spins and are non-Frenkel-like, while the bright excitons have mixed
spin configurations. The intrinsic lifetime of excitons can be over
one nanosecond, suitable for quantum information processes. Tensile
strains and bending manifestly modulate the absorption spectra and
magneto-optical properties of CrI3 nanoribbons within a
technologically important photon energy range of âź1.0â2.0
eV. The CrI3 nanoribbons can be used in 1D or 2D magnetic
storage nanodevices, tunable magnetic optoelectronics, and spin-based
quantum information controls
Accurate Complete Basis Set Extrapolation of Direct Random Phase Correlation Energies
The
direct random phase approximation (dRPA) is a promising way
to obtain improvements upon the standard semilocal density functional
results in many aspects of computational chemistry. In this paper,
we address the slow convergence of the calculated dRPA correlation
energy with the increase of the quality and size of the popular Gaussian-type
Dunningâs correlation consistent aug-cc-pV<i>X</i>Z split valence atomic basis set family. The cardinal number <i>X</i> controls the size of the basis set, and we use <i>X</i> = 3â6 in this study. It is known that even the
very expensive <i>X</i> = 6 basis sets lead to large errors
for the dRPA correlation energy, and thus complete basis set extrapolation
is necessary. We study the basis set convergence of the dRPA correlation
energies on a set of 65 hydrocarbon isomers from CH<sub>4</sub> to
C<sub>6</sub>H<sub>6</sub>. We calculate the iterative density fitted
dRPA correlation energies using an efficient algorithm based on the
CC-like form of the equations using the self-consistent HF orbitals.
We test the popular inverse cubic, the optimized exponential, and
inverse power formulas for complete basis set extrapolation. We have
found that the optimized inverse power based extrapolation delivers
the best energies. Further analysis showed that the optimal exponent
depends on the molecular structure, and the most efficient two-point
energy extrapolations that use <i>X</i> = 3 and 4 can be
improved considerably by considering the atomic composition and hybridization
states of the atoms in the molecules. Our results also show that the
optimized exponents that yield accurate <i>X</i> = 3 and
4 extrapolated dRPA energies for atoms or small molecules might be
inaccurate for larger molecules
Adiabatic Connection without Coupling Constant Integration
Using a second-order approximation
to Random Phase Approximation
renormalized (RPAr) many-body perturbation theory for the interacting
densityâdensity response function, we have developed a so-called
higher-order terms (HOT) approximation for the correlation energy.
In combination with the first-order RPAr correction, our new method
faithfully captures the infinite-order correlation for a given exchange-correlation
kernel, yielding errors of the total correlation energy on the order
of 1% or less for most systems. For exchange-like kernels, our new
method has the further benefit that the coupling-strength integration
can be completely eliminated resulting in a modest reduction in computational
cost compared to the traditional approach. When the correlation energy
is accurately reproduced by the HOT approximation, structural properties
and energy differences are also accurately reproduced, as we demonstrate
for several periodic solids and some molecular systems. Energy differences
involving fragmentation are challenging for the HOT method, however,
due to errors that may not cancel between a composite system and its
constituent pieces
A meta-GGA Made Free of the Order of Limits Anomaly
We have improved the revised TaoâPerdewâStaroverovâScuseria
(revTPSS) meta-generalized gradient approximation (GGA) in order to
remove the order of limits anomaly in its exchange energy. The revTPSS
meta-GGA recovers the second-order gradient expansion for a wide range
of densities and therefore provides excellent atomization energies
and lattice constants. For other properties of materials, however,
even the revTPSS does not give the desired accuracy. The revTPSS does
not perform as well as expected for the energy differences between
different geometries for the same molecular formula and for the related
nonbarrier height chemical reaction energies. The same order of limits
problem might lead to inaccurate energy differences between different
crystal structures and to inaccurate cohesive energies of insulating
solids. Here we show a possible way to remove the order of limits
anomaly with a weighted difference of the revTPSS exchange between
the slowly varying and iso-orbitals (one- or two-electron) limits.
We show that the new regularized (regTPSS) gives atomization energies
comparable to revTPSS and preserves the accurate lattice constants
as well. For other properties, the regTPSS gives at least the same
performance as the revTPSS or TPSS meta-GGAs
Construction and Application of a New Dual-Hybrid Random Phase Approximation
The
direct random phase approximation (dRPA) combined with KohnâSham
reference orbitals is among the most promising tools in computational
chemistry and applicable in many areas of chemistry and physics. The
reason for this is that it scales as <i>N</i><sup>4</sup> with the system size, which is a considerable advantage over the
accurate ab initio wave function methods like standard coupled-cluster.
dRPA also yields a considerably more accurate description of thermodynamic
and electronic properties than standard density-functional theory
methods. It is also able to describe strong static electron correlation
effects even in large systems with a small or vanishing band gap missed
by common single-reference methods. However, dRPA has several flaws
due to its self-correlation error. In order to obtain accurate and
precise reaction energies, barriers and noncovalent intra- and intermolecular
interactions, we construct a new dual-hybrid dRPA (hybridization of
exact and semilocal exchange in both the energy and the orbitals)
and test the performance of this new functional on isogyric, isodesmic,
hypohomodesmotic, homodesmotic, and hyperhomodesmotic reaction classes.
We also use a test set of 14 DielsâAlder reactions, six atomization
energies (AE6), 38 hydrocarbon atomization energies, and 100 reaction
barrier heights (DBH24, HT-BH38, and NHT-BH38). For noncovalent complexes,
we use the NCCE31 and S22 test sets. To test the intramolecular interactions,
we use a set of alkane, cysteine, phenylalanine-glycine-glycine tripeptide,
and monosaccharide conformers. We also discuss the delocalization
and static correlation errors. We show that a universally accurate
description of chemical properties can be provided by a large, 75%
exact exchange mixing both in the calculation of the reference orbitals
and the final energy
Construction of a Spin-Component Scaled Dual-Hybrid Random Phase Approximation
Recently, we have constructed a dual-hybrid
direct random phase
approximation method, called dRPA75, and demonstrated its good performance
on reaction energies, barrier heights, and noncovalent interactions
of main-group elements. However, this method has also shown significant
but quite systematic errors in the computed atomization energies.
In this paper, we suggest a constrained spin-component scaling formalism
for the dRPA75 method (SCS-dRPA75) in order to overcome the large
error in the computed atomization energies, preserving the good performance
of this method on spin-unpolarized systems at the same time. The SCS-dRPA75
method with the aug-cc-pVTZ basis set results in an average error
lower than 1.5 kcal mol<sup>â1</sup> for the entire <i>n</i>-homodesmotic hierarchy of hydrocarbon reactions (RC0âRC5
test sets). The overall performance of this method is better than
the related direct random phase approximation-based double-hybrid
PWRB95 method on open-shell systems of main-group elements (from the
GMTKN30 database) and comparable to the best <i>O</i>(<i>N</i><sup>4</sup>)-scaling opposite-spin second-order perturbation
theory-based double-hybrid methods like PWPB95-D3 and to the <i>O</i>(<i>N</i><sup>5</sup>)-scaling RPAX2@PBEx method,
which also includes exchange interactions. Furthermore, it gives well-balanced
performance on many types of barrier heights similarly to the best <i>O</i>(<i>N</i><sup>5</sup>)-scaling second-order perturbation
theory-based or spin-component scaled second-order perturbation theory-based
double-hybrid methods such as XYG3 or DSD-PBEhB95. Finally, we show
that the SCS-dRPA75 method has reduced self-interaction and delocalization
errors compared to the parent dRPA75 method and a slightly smaller
static correlation error than the related PWRB95 method
Accurate, Precise, and Efficient Theoretical Methods To Calculate AnionâĎ Interaction Energies in Model Structures
A correct
description of the anionâĎ interaction is essential for
the design of selective anion receptors and channels and important
for advances in the field of supramolecular chemistry. However, it
is challenging to do accurate, precise, and efficient calculations
of this interaction, which are lacking in the literature. In this
article, by testing sets of 20 binary anionâĎ complexes
of fluoride, chloride, bromide, nitrate, or carbonate ions with hexafluorobenzene,
1,3,5-trifluorobenzene, 2,4,6-trifluoro-1,3,5-triazine, or 1,3,5-triazine
and 30 ternary ĎâanionâĎⲠsandwich
complexes composed from the same monomers, we suggest domain-based
local-pair natural orbital coupled cluster energies extrapolated to
the complete basis-set limit as reference values. We give a detailed
explanation of the origin of anionâĎ interactions, using
the permanent quadrupole moments, static dipole polarizabilities,
and electrostatic potential maps. We use symmetry-adapted perturbation
theory (SAPT) to calculate the components of the anionâĎ
interaction energies. We examine the performance of the direct random
phase approximation (dRPA), the second-order screened exchange (SOSEX),
local-pair natural-orbital (LPNO) coupled electron pair approximation
(CEPA), and several dispersion-corrected density functionals (including
generalized gradient approximation (GGA), meta-GGA, and double hybrid
density functional). The LPNO-CEPA/1 results show the best agreement
with the reference results. The dRPA method is only slightly less
accurate and precise than the LPNO-CEPA/1, but it is considerably
more efficient (6â17 times faster) for the binary complexes
studied in this paper. For 30 ternary ĎâanionâĎâ˛
sandwich complexes, we give dRPA interaction energies as reference
values. The double hybrid functionals are much more efficient but
less accurate and precise than dRPA. The dispersion-corrected double
hybrid PWPB95âD3Â(BJ) and B2PLYPâD3Â(BJ) functionals perform
better than the GGA and meta-GGA functionals for the present test
set
Performance of meta-GGA Functionals on General Main Group Thermochemistry, Kinetics, and Noncovalent Interactions
Among the computationally efficient semilocal density
functionals
for the exchange-correlation energy, meta-generalized-gradient approximations
(meta-GGAs) are potentially the most accurate. Here, we assess the
performance of three new meta-GGAs (revised TaoâPerdewâStaroverovâScuseria
or revTPSS, regularized revTPSS or regTPSS, and meta-GGA made simple
or MGGA_MS), within and beyond their âcomfort zones,â
on Grimmeâs big test set of main-group molecular energetics
(thermochemistry, kinetics, and noncovalent interactions). We compare
them against the standard PerdewâBurkeâErnzerhof (PBE)
GGA, TPSS, and Minnesota M06L meta-GGAs, and Becke-3-LeeâYangâParr
(B3LYP) hybrid of GGA with exact exchange. The overall performance
of these three new meta-GGA functionals is similar. However, dramatic
differences occur for different test sets. For example, M06L and MGGA_MS
perform best for the test sets that contain noncovalent interactions.
For the 14 DielsâAlder reaction energies in the âdifficultâ
DARC subset, the mean absolute error ranges from 3 kcal mol<sup>â1</sup> (MGGA_MS) to 15 kcal mol<sup>â1</sup> (B3LYP), while for
some other reaction subsets the order of accuracy is reversed; more
generally, the tested new semilocal functionals outperform the standard
B3LYP for ring reactions. Some overall improvement is found from long-range
dispersion corrections for revTPSS and regTPSS but not for MGGA_MS.
Formal and universality criteria for the functionals are also discussed