Quantum algorithm for simulating molecular vibrational excitations.

The excitation of vibrational modes in molecules affects the outcome of chemical reactions, for example by providing molecules with sufficient energy to overcome activation barriers. In this work, we introduce a quantum algorithm for simulating molecular vibrational excitations during vibronic transitions. We discuss how a special-purpose quantum computer can be programmed with molecular data to optimize a vibronic process such that desired modes get excited during the transition. We investigate the effect of such excitations on selective bond dissociation in pyrrole and butane during photochemical and mechanochemical vibronic transitions. The results are discussed with respect to experimental observations and classical simulations. We also introduce quantum-inspired classical algorithms for simulating molecular vibrational excitations in special cases where only a limited number of modes are of interest.

Density-functional tight-binding investigation of the structure, stability and material properties of nickel hydroxide nanotubes.

Nickel hydroxide is a material composed of two-dimensional layers that can be rolled up to form cylindrical nanotubes belonging to a class of inorganic metal hydroxide nanotubes that are candidates for applications in catalysis, energy storage, and microelectronics. The stabilities and other properties of this class of inorganic nanotubes have not yet been investigated in detail. The present study uses self-consistent-charge density-functional tight-binding calculations to examine the stabilities, mechanical properties, and electronic properties of nickel hydroxide nanotubes along with the energetics associated with the adsorption of water by these systems. The tight-binding model was parametrized for this system based on the results of first-principles calculations. The stabilities of the nanotubes were examined by calculating strain energies and performing molecular dynamics simulations. The results indicate that single-walled nickel hydroxide nanotubes are stable at room temperature, which is consistent with experimental investigations. The nanotubes possess size-dependent mechanical properties that are similar in magnitude to those of other inorganic nanotubes. The electronic properties of the nanotubes were also found to be size-dependent and small nickel oxyhydroxide nanotubes are predicted to be semiconductors. Despite this size-dependence, both the mechanical and electronic properties were found to be almost independent of the helical structure of the nanotubes. The calculations also show that water molecules have higher adsorption energies when binding to the interior of the nickel hydroxide nanotubes when compared to adsorption in nanotubes formed from other two-dimensional materials such as graphene. The increased adsorption energy is due to the hydrophilic nature of nickel hydroxide. Due to the broad applications of nickel hydroxide, the nanotubes investigated here are also expected to be used in catalysis, electronics, and clean energy production.

Molecular structure and interactions of water intercalated in nickel hydroxide.

The structure and properties of α-Ni(OH)2 containing water and nitrate have been investigated computationally. The adsorption of water molecules on the Ni(OH)2 surface is also investigated to provide insight into the nature of the water-Ni(OH)2 interactions. The spectroscopic and dynamical behaviour of the intercalated species has been characterized and used to explain experimental findings reported for this material. The results presented here indicate that the water molecules interact non-covalently with Ni(OH)2, with a binding energy that is comparable in magnitude with that of the water dimer hydrogen bond. The presence of the intercalated species increases the distance between the Ni(OH)2 layers such that the interlayer interactions are negligible. The weakening of the interlayer interactions facilitates the horizontal displacement of the layers relative to one another, providing a possible origin for stacking faults observed in α-Ni(OH)2. Comparison of the vibrational frequencies calculated here with the experimental spectra confirms that α-Ni(OH)2 containing only water molecules can be synthesized. The structures of the water molecules intercalated in α-Ni(OH)2 were found to be analogous to those absorbed in γ-NiOOH, while the water-layer interactions are stronger in γ-NiOOH. The results presented here characterize the structure and interactions of water intercalated in nickel hydroxides and also provide insights into the effects of intercalated water on the properties of layered metal hydroxides.

Effects of reduced dimensionality on the properties of magnesium hydroxide and calcium hydroxide nanostructures.

Metal hydroxides are a class of layered materials that contain two-dimensional metal hydroxide layers that can be isolated to form layered nanostructures. In this work, density functional theory (DFT) and self-consistent-charge density-functional tight-binding (SCC-DFTB) methods have been used to investigate the properties of magnesium hydroxide and calcium hydroxide nanostructures. The properties of single layer and multi layer structures with up to 10 metal hydroxide sheets and nanoparticles containing more than 2000 atoms have been calculated and compared with the bulk properties of these systems. The accuracy of the DFT methods employed and SCC-DFTB parameters developed in this study were validated against available experimental data. The results of the calculations indicate that significant differences exist between the properties of the nanostructures and the corresponding bulk values. In particular, the interlayer binding energies, electronic band gaps, and spectroscopic features are size-dependent and tend to converge to the bulk values as the size of the nanosystem is increased. The calculated binding energies and shear moduli show that all nanostructures are mechanically stable, in agreement with the experimental reports; although, their stabilities may be affected by the presence of intercalated species. Energy decomposition analyses reveal that the intralayer interactions in the investigated systems are predominantly electrostatic in nature, while the interlayer interactions are dominated by dispersion and polarization components. The results presented here quantify various properties of magnesium hydroxide and calcium hydroxide nanostructures, and could be used to understand the properties of other nanosystems containing layers of metal hydroxides in their structure.

Performance of density-functional tight-binding models in describing hydrogen-bonded anionic-water clusters.

Density-functional tight-binding (DFTB) models are computationally efficient approximations to density-functional theory that have been shown to predict reliable structural and energetic properties for various systems. In this work, the reliability and accuracy of the self-consistent-charge DFTB model and its recent extension(s) in predicting the structures, binding energies, charge distributions, and vibrational frequencies of small water clusters containing polyatomic anions of the Hofmeister series (carbonate, sulfate, hydrogen phosphate, acetate, nitrate, perchlorate, and thiocyanate) have been carefully and systematically evaluated on the basis of high-level ab initio quantum-chemistry [MP2/aug-cc-pVTZ and CCSD(T)/aug-cc-pVQZ] reference data. Comparison with available experimental data has also been made for further validation. The self-consistent-charge DFTB model, and even more so its recent extensions, are shown to properly account for the structural properties, energetics, intermolecular polarization, and spectral signature of hydrogen-bonding in anionic water clusters at a fraction of the computational cost of ab initio quantum-chemistry methods. This makes DFTB models candidates of choice for investigating much larger systems such as seeded water droplets, their structural properties, formation thermodynamics, and infrared spectra.

Quantum Simulation of Molecules in Solution.

Quantum chemical calculations on quantum computers have been focused mostly on simulating molecules in the gas phase. Molecules in liquid solution are, however, most relevant for chemistry. Continuum solvation models represent a good compromise between computational affordability and accuracy in describing solvation effects within a quantum chemical description of solute molecules. In this work, we extend the variational quantum eigensolver to simulate solvated systems using the polarizable continuum model. To account for the state dependent solute-solvent interaction we generalize the variational quantum eigensolver algorithm to treat non-linear molecular Hamiltonians. We show that including solvation effects does not impact the algorithmic efficiency. Numerical results of noiseless simulations for molecular systems with up to 12 spin-orbitals (qubits) are presented. Furthermore, calculations performed on a simulated noisy quantum hardware (IBM Q, Mumbai) yield computed solvation free energies in fair agreement with the classical calculations.

Quantum Algorithm for Simulating Single-Molecule Electron Transport.

An accurate description of electron transport at a molecular level requires a precise treatment of quantum effects. These effects play a crucial role in determining the electron transport properties of single molecules, which can be challenging to simulate classically. Here we introduce a quantum algorithm to efficiently calculate electronic current through single-molecule junctions in the weak-coupling regime. We show that a quantum computer programmed to simulate vibronic transitions between different charge states of a molecule can be used to compute electron-transfer rates and electronic current. In the harmonic approximation, the algorithm can be implemented using Gaussian boson sampling devices, which are a near-term platform for photonic quantum computing. We apply the algorithm to simulate the current and conductance of a magnesium porphine molecule. The algorithm provides a means for better understanding the mechanism of electron transport at a molecular level, which paves the way for building practical molecular electronic devices.

Point processes with Gaussian boson sampling.

Random point patterns are ubiquitous in nature, and statistical models such as point processes, i.e., algorithms that generate stochastic collections of points, are commonly used to simulate and interpret them. We propose an application of quantum computing to statistical modeling by establishing a connection between point processes and Gaussian boson sampling, an algorithm for photonic quantum computers. We show that Gaussian boson sampling can be used to implement a class of point processes based on hard-to-compute matrix functions which, in general, are intractable to simulate classically. We also discuss situations where polynomial-time classical methods exist. This leads to a family of efficient quantum-inspired point processes, including a fast classical algorithm for permanental point processes. We investigate the statistical properties of point processes based on Gaussian boson sampling and reveal their defining property: like bosons that bunch together, they generate collections of points that form clusters. Finally, we analyze properties of these point processes for homogeneous and inhomogeneous state spaces, describe methods to control cluster location, and illustrate how to encode correlation matrices.

Parameterization of Halogens for the Density-Functional Tight-Binding Description of Halide Hydration.

Parameter sets of the self-consistent-charge density-functional tight-binding model with and without its third-order extension have been developed to describe the interatomic interactions of halogen elements (X = Cl, Br, I) with hydrogen and oxygen, with the ultimate goal of investigating halide hydration with this approach. The reliability and accuracy of the model with these newly developed parameters has been evaluated by comparing the structural, energetic, and vibrational properties of small molecules containing halogen atoms with those obtained by means of standard density-functional theory. Furthermore, the newly parametrized model is found to predict equilibrium geometries, binding energies, and vibrational frequencies for small aqueous clusters containing halogen anions, X(-)(H2O)n (n = 1-4), in good agreement with those calculated with density-functional theory and high-level ab initio quantum chemistry and with available experimental data. This demonstrates that the newly parametrized models might be a method of choice for investigating halide hydration in larger clusters.

Computational investigation of the hydration of alkyl diammonium chlorides and their effect on THF/water phase separation.

The solvation behavior of alkyl diammonium chlorides of varying alkyl chain length and the molecular details of their effect on the salting-out of organic molecules in aqueous phase have been investigated by classical molecular dynamics simulations. More specifically, systems containing water, tetrahydrofuran (THF), and their mixtures with α,ω-alkyl diammonium chlorides [H3N(CH2)nNH3]Cl2 (n = 2, 4, 6, 8, and 10) were simulated at ambient temperature and pressure. Various force fields were tested and one was chosen based on its ability to reproduce the physical properties of the pure THF solution and its mixture with water. Structural and thermodynamic analyses of the simulated salt-solvent mixtures reveal different extents of hydration of the dications depending on the alkyl chain length and indicate that the hydrophobic interactions between the dication alkyl chain and organic molecules play a key role in the solvation of the latter species. In fact, shorter dications are shown to promote THF/water phase separation, in agreement with previous experimental findings.