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

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

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

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 CH₄ 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 CH₄ 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 (Pd²⁺–O^2–) in PdO. The charges in the CH₃ 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 Pd²⁺–O^2– 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 (H₃C···Pd···H)^⧧ transition states. On O*-saturated Pd surfaces, C–H bonds cleave homolytically on O*–O* pairs to form radical-like CH₃ species and nearly formed O–H bonds at a transition state (O*···CH₃^•···*OH)^⧧ that is looser and higher in enthalpy than on bare Pd surfaces. On PdO surfaces, site pairs consisting of exposed Pd²⁺ and vicinal O^2–, Pd_ox–O_ox , cleave C–H bonds heterolytically via σ-bond metathesis, with Pd²⁺ adding to the C–H bond, while O^2– abstracts the H-atom to form a four-center (H₃C^δ−···Pd_ox···H^δ+···O_ox)^⧧ transition state without detectable Pd_ox reduction. The latter is much more stable than transition states on *–* and O*–O* pairs and give rise to a large increase in CH₄ 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

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)

Aromatic aldehydes as tuneable and ppm level potent promoters for zeolite catalysed methanol dehydration to DME

Dimethyl ether (DME) is a valuable chemical intermediate and renewable fuel that can be made, via methanol, from many sources of carbon, including carbon dioxide and biomass. Benzaldehyde and its derivatives have been found to be promoters for zeolite catalysed methanol dehydration to DME at low temperature (110 to 150 oC). For the 3-dimensional medium pore zeolite H-ZSM-5 (MFI) the promotion is readily reversible and the potency of the promoter can be tuned by varying the substituent on the aromatic ring of the aldehyde. The most potent promoters are active at concentrations as low as 1 ppm relative to methanol. High throughput experimentation (HTE) is used to screen and rank potential promoters and catalysts and to collect high quality kinetic data for the most promising candidates discovered. The catalytic data and in-situ FT-IR-MS experiments combined with molecular modelling studies indicate a mechanism involving competitive adsorption of the aldehyde promoter on a Brønsted acid (BA) site, followed by reaction with methanol to give a hemi-acetal intermediate. Loss of water from the hemi-acetal intermediate generates a transient and highly reactive methyl oxonium species, [ArC(H)(=O-Me)]+, which then directly reacts with methanol via a SN2 mechanism to give DME and regenerate the aldehyde promoter and BA site. The methyl oxonium species is stabilized by electron-donating groups on the aromatic ring and the solvent like effect of the zeolite pore walls. Molecular descriptors were calculated by molecular modelling for the 22 aromatic aldehyde promoters tested. Multivariate linear regression analysis was used to build an interpretable model for aldehyde promotional activity in H-ZSM-5 and in another 3-dimensional medium pore zeolite, H-ZSM-11 (MEL)

Different Product Distributions and Mechanistic Aspects of the Hydrodeoxygenation of m‑Cresol over Platinum and Ruthenium Catalysts

Experimental measurements of the conversion of m-cresol over Pt and Ru/SiO₂ catalysts show very different product distributions, even when the reaction is conducted at similarly low conversions and the same operating conditions (300 °C, 1 atm). That is, although ring hydrogenation to 3-methylcyclohexanone is dominant over Pt, deoxygenation to toluene and C–C cleavage to C₁–C₅ hydrocarbons prevail over Ru. For understanding the differences in reaction mechanisms responsible for this contrasting behavior, the conversion of m-cresol over the Pt(111) and Ru(0001) surfaces has been analyzed using density functional theory (DFT) methods. The DFT results show that the direct dehydroxylation of m-cresol is unfavorable over the Pt(111) surface with an energy barrier of 242 kJ/mol. In turn, the calculations suggest that the reaction could proceed through a keto tautomer intermediate, which undergoes hydrogenation of the carbonyl group followed by dehydration to form toluene and water. At the same time, a low energy barrier for the ring hydrogenation path toward 3-methylcyclohexanone compared to the energy barrier for the deoxygenation path toward toluene over the Pt(111) surface is in agreement with the experimental observations, which show that 3-methylcyclohexanone is the dominant product over Pt/SiO₂ at low conversions. By contrast, the direct dehydroxylation of m-cresol becomes more favorable than the tautomerization route over the more oxophilic Ru(0001) surface. In this case, the deoxygenation path exhibits an energy barrier lower than that for the ring hydrogenation, which is also in agreement with experimental results that show higher selectivity to the deoxygenation product toluene. Finally, it is proposed that a partially unsaturated hydrocarbon surface species C₇H₇* is formed during the direct dehydroxylation of m-cresol over Ru(0001), becoming the crucial intermediate for the C–C bond breaking products C₁–C₅ hydrocarbons, which are observed experimentally over the Ru/SiO₂ catalyst