19 research outputs found
Identifying challenges towards practical quantum advantage through resource estimation: the measurement roadblock in the variational quantum eigensolver
Recent advances in Noisy Intermediate-Scale Quantum (NISQ) devices have
brought much attention to the potential of the Variational Quantum Eigensolver
(VQE) and related techniques to provide practical quantum advantage in
computational chemistry. However, it is not yet clear whether such algorithms,
even in the absence of device error, could achieve quantum advantage for
systems of practical interest and how large such an advantage might be. To
address these questions, we have performed an exhaustive set of benchmarks to
estimate number of qubits and number of measurements required to compute the
combustion energies of small organic molecules to within chemical accuracy
using VQE as well as state-of-the-art classical algorithms. We consider several
key modifications to VQE, including the use of Frozen Natural Orbitals, various
Hamiltonian decomposition techniques, and the application of fermionic marginal
constraints. Our results indicate that although Frozen Natural Orbitals and
low-rank factorizations of the Hamiltonian significantly reduce the qubit and
measurement requirements, these techniques are not sufficient to achieve
practical quantum computational advantage in the calculation of organic
molecule combustion energies. This suggests that new approaches to estimation
leveraging quantum coherence, such as Bayesian amplitude estimation
[arxiv:2006.09350, arxiv:2006.09349], may be required in order to achieve
practical quantum advantage with near-term devices. Our work also highlights
the crucial role that resource and performance assessments of quantum
algorithms play in identifying quantum advantage and guiding quantum algorithm
design.Comment: 27 pages, 18 figure
Fieldlike and antidamping spin-orbit torques in as-grown and annealed Ta/CoFeB/MgO layers
We present a comprehensive study of the current-induced spin-orbit torques in
perpendicularly magnetized Ta/CoFeB/MgO layers. The samples were annealed in
steps up to 300 degrees C and characterized using x-ray absorption
spectroscopy, transmission electron microscopy, resistivity, and Hall effect
measurements. By performing adiabatic harmonic Hall voltage measurements, we
show that the transverse (field-like) and longitudinal (antidamping-like)
spin-orbit torques are composed of constant and magnetization-dependent
contributions, both of which vary strongly with annealing. Such variations
correlate with changes of the saturation magnetization and magnetic anisotropy
and are assigned to chemical and structural modifications of the layers. The
relative variation of the constant and anisotropic torque terms as a function
of annealing temperature is opposite for the field-like and antidamping
torques. Measurements of the switching probability using sub-{\mu}s current
pulses show that the critical current increases with the magnetic anisotropy of
the layers, whereas the switching efficiency, measured as the ratio of magnetic
anisotropy energy and pulse energy, decreases. The optimal annealing
temperature to achieve maximum magnetic anisotropy, saturation magnetization,
and switching efficiency is determined to be between 240 degrees and 270
degrees C
3-Center-4-Electron Bonding in [(Silox)2ModNtBu]2(m-Hg) Controls Reactivity while Frontier Orbitals Permit a Dimolybdenum p-Bond Energy Estimate
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De novo prediction of the ground state structure of transition metal complexes.
One of the main goals of computational methods is to identify reasonable geometries for target materials. Organometallic complexes have been investigated in this dissertation research, entailing a significant challenge based on transition metal diversity and the associated complexity of the ligands. A large variety of theoretical methods have been employed to determine ground state geometries of organometallic species. An impressive number of transition metals entailing diverse isomers (e.g., geometric, spin, structural and coordination), different coordination numbers, oxidation states and various numbers of electrons in d orbitals have been studied. Moreover, ligands that are single, double or triple bonded to the transition metal, exhibiting diverse electronic and steric effects, have been investigated. In this research, a novel de novo scheme for structural prediction of transition metal complexes was developed, tested and shown to be successful
Hydrogen Physisorption on the Organic Linker in Metal Organic Frameworks:Â Ab Initio Computational Study
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Stability Studies of Transition-Metal Linkage Isomers Using Quantum Mechanical Methods, Groups 11 and 12 Transition Metals
Article discussing the stability studies of transition-metal linkage isomers using quantum mechanical methods and groups 11 and 12 transition metals
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Low-Coordinate Chromium Siloxides: The "Box" [Cr(μ-OSitBu3)]4, Distorted Trigonal [(tBu3SiO)3Cr] [Na(benzene)] and [(tBu3SiO)3Cr] [Na(dibenzo-18-c-6)], and Trigonal (tBu3SiO)3Cr
Article discussing low-coordinate chromium siloxides and the "box" [Cr(μ-Cl)(μ-OSiᵗBu₃)]₄, distorted trigonal [(ᵗBu₃SiO)₃Cr][Na(benzene)] and [(ᵗBu₃SiO)₃Cr][Na(dibenzo-18-c-6)], and trigonal (ᵗBu₃SiO)₃Cr
Consequences of Metal–Oxide Interconversion for C–H Bond Activation during CH<sub>4</sub> Reactions on Pd Catalysts
Mechanistic assessments based on
kinetic and isotopic methods combined
with density functional theory are used to probe the diverse pathways
by which C–H bonds in CH<sub>4</sub> react on bare Pd clusters,
Pd cluster surfaces saturated with chemisorbed oxygen (O*), and PdO
clusters. C–H activation routes change from oxidative addition
to H-abstraction and then to σ-bond metathesis with increasing
O-content, as active sites evolve from metal atom pairs (*–*)
to oxygen atom (O*–O*) pairs and ultimately to Pd cation-lattice
oxygen pairs (Pd<sup>2+</sup>–O<sup>2–</sup>) in PdO.
The charges in the CH<sub>3</sub> and H moieties along the reaction
coordinate depend on the accessibility and chemical state of the Pd
and O centers involved. Homolytic C–H dissociation prevails
on bare (*–*) and O*-covered surfaces (O*–O*), while
C–H bonds cleave heterolytically on Pd<sup>2+</sup>–O<sup>2–</sup> pairs at PdO surfaces. On bare surfaces, C–H
bonds cleave via oxidative addition, involving Pd atom insertion into
the C–H bond with electron backdonation from Pd to C–H
antibonding states and the formation of tight three-center (H<sub>3</sub>C···Pd···H)<sup>⧧</sup> transition states. On O*-saturated Pd surfaces, C–H bonds
cleave homolytically on O*–O* pairs to form radical-like CH<sub>3</sub> species and nearly formed O–H bonds at a transition
state (O*···CH<sub>3</sub><sup>•</sup>···*OH)<sup>⧧</sup> that is looser and higher in enthalpy than on bare
Pd surfaces. On PdO surfaces, site pairs consisting of exposed Pd<sup>2+</sup> and vicinal O<sup>2–</sup>, Pd<sub>ox</sub>–O<sub>ox</sub> , cleave C–H bonds heterolytically via σ-bond
metathesis, with Pd<sup>2+</sup> adding to the C–H bond, while
O<sup>2–</sup> abstracts the H-atom to form a four-center (H<sub>3</sub>C<sup>δ−</sup>···Pd<sub>ox</sub>···H<sup>δ+</sup>···O<sub>ox</sub>)<sup>⧧</sup> transition state without detectable Pd<sub>ox</sub> reduction. The latter is much more stable than transition states
on *–* and O*–O* pairs and give rise to a large increase
in CH<sub>4</sub> oxidation turnover rates at oxygen chemical potentials
leading to Pd to PdO transitions. These distinct mechanistic pathways
for C–H bond activation, inferred from theory and experiment,
resemble those prevalent on organometallic complexes. Metal centers
present on surfaces as well as in homogeneous complexes act as both
nucleophile and electrophile in oxidative additions, ligands (e.g.,
O* on surfaces) abstract H-atoms via reductive deprotonation of C–H
bonds, and metal–ligand pairs, with the pair as electrophile
and the metal as nucleophile, mediate σ-bond metathesis pathways
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3-Center-4-Electron Bonding in [(silox)2Mo=NtBu]2(μ-Hg) Controls Reactivity while Frontier Orbitals Permit a Dimolybdenum π-Bond Energy Estimate
Article describing research on 3-center-4-electron bonding in [(silox)2Mo=NtBu]2(mu-Hg)
Diffusion Mechanisms and Preferential Dynamics of Promoter Molecules in ZSM-5 Zeolite
The diffusion in ZSM-5 zeolite of methanol and of two series of promoters of the methanol to dimethyl
ether reaction (linear methyl esters, benzaldehyde, 4-n-alkyl benzaldehydes) has been studied using
classical molecular dynamics in the NVT ensemble. Whereas promoter diffusion coefficients decrease
with increasing alkyl chain length in methyl esters, the aromatic aldehyde promoters all have similar
diffusion coefficients. The lowest diffusion coefficient is that of benzaldehyde. All the promoters
exhibit a preference for moving in the straight pore, a preference that is most pronounced for the
4-n-alkyl benzaldehydes and least for the longest aliphatic esters. A novel diffusion mechanism, a
molecular ’3-point turn’, is observed. The diffusion coefficient of methanol is larger than that of all
the promoters. The more catalytically active aromatic aldehyde promoters limit methanol diffusion less than the aliphatic esters