Publications

The variational quantum eigensolver (VQE) is a widely employed method to solve electronic structure problems in the current noisy intermediate-scale quantum (NISQ) devices. However, due to inherent noise in the NISQ devices, VQE results on NISQ devices often deviate significantly from the results obtained on noiseless statevector simulators or traditional classical computers. The iterative nature of VQE further amplifies the errors in each loop. Recent works have explored ways to integrate deep neural networks (DNN) with VQE to mitigate iterative errors, albeit primarily limited to the noiseless statevector simulators. In this work, we trained DNN models across various quantum circuits and examined the potential of two DNN-VQE approaches, DNN1 and DNNF, for predicting the ground state energies of small molecules in the presence of device noise. We carefully examined the accuracy of the DNN1, DNNF, and VQE methods on both noisy simulators and real quantum devices by considering different ansatzes of varying qubit counts and circuit depths. Our results illustrate the advantages and limitations of both VQE and DNN-VQE approaches. Notably, both DNN1 and DNNF methods consistently outperform the standard VQE method in providing more accurate ground state energies in noisy environments. However, despite being more accurate than VQE, the energies predicted using these methods on real quantum hardware remain meaningful only at reasonable circuit depths (depth = 15, gates = 21). At higher depths (depth = 83, gates = 112), they deviate significantly from the exact results. Additionally, we find that DNNF does not offer any notable advantage over VQE in terms of speed. Consequently, our study recommends DNN1 as the preferred method for obtaining quick and accurate ground state energies of molecules on current quantum hardware, particularly for quantum circuits with lower depth and fewer qubits.

‘Hot atoms’, atoms in their excited states, transfer their energy to the surrounding atmosphere through collisions. This process (known as thermalization) plays a crucial role in various astrophysical and atmospheric processes. Thermalization of hot atoms is mainly governed by the amount of species present in the surrounding atmosphere and the collision cross-sections between the hot atoms and surrounding species. In this work, we investigated the elastic and inelastic collisions between hot oxygen atoms and neutral N2 molecules, relevant to oxygen gas escape from the Martian atmosphere and for characterizing the chemical reactions in hypersonic flows. We conducted a series of quantum scattering calculations between various isotopes of O(3P) atoms and N2 molecules across a range of collision energies (0.3–4 eV), and computed both their differential and collision cross-sections using quantum time-independent coupled-channel approach. Our differential cross-section results indicate a strong preference for forward scattering over sideways or backward scattering, and this anisotropy in scattering is further pronounced at higher collision energies. By comparing the cross-sections of three oxygen isotopes, we find that the heavier isotopes consistently have larger collision cross-sections than the lighter isotopes. As a whole, this study contributes to a better understanding of the energy distribution and thermalization processes of hot atoms within atmospheric environments. Specifically, the cross-sectional data presented in this work is directly useful in improving the accuracy of energy relaxation modelling of O and N2 collisions over the Mars and Venus atmospheres.

‘Hot atoms’, atoms in their excited states, transfer their energy to the surrounding atmosphere through collisions. This process (known as thermalization) plays a crucial role in various astrophysical and atmospheric processes. Thermalization of hot atoms is mainly governed by the amount of species present in the surrounding atmosphere and the collision cross-sections between the hot atoms and surrounding species. In this work, we investigated the elastic and inelastic collisions between hot oxygen atoms and neutral N2 molecules, relevant to oxygen gas escape from the Martian atmosphere and for characterizing the chemical reactions in hypersonic flows. We conducted a series of quantum scattering calculations between various isotopes of O(3P) atoms and N2 molecules across a range of collision energies (0.3–4 eV), and computed both their differential and collision cross-sections using quantum time-independent coupled-channel approach. Our differential cross-section results indicate a strong preference for forward scattering over sideways or backward scattering, and this anisotropy in scattering is further pronounced at higher collision energies. By comparing the cross-sections of three oxygen isotopes, we find that the heavier isotopes consistently have larger collision cross-sections than the lighter isotopes. As a whole, this study contributes to a better understanding of the energy distribution and thermalization processes of hot atoms within atmospheric environments. Specifically, the cross-sectional data presented in this work is directly useful in improving the accuracy of energy relaxation modelling of O and N2 collisions over the Mars and Venus atmospheres.

The remediation of per- and polyfluoroalkyl substances (PFAS) in water continues to garner significant attention due to their environmental persistence and adverse health effects. Among the various PFAS remediation methods, photoinduced approaches have recently emerged as promising techniques for the degradation of these persistent contaminants. However, many questions remain unanswered regarding the detailed mechanisms in these photoinduced methods as well as the best ways to leverage these approaches for PFAS degradation. In this review, we provide an update on recent experimental and theoretical developments in photoinduced PFAS degradation techniques over the past 2–5 years. We conclude with a perspective of promising research directions in this vibrant area and give recommendations on future experimental and computational approaches needed to further advance these photoinduced remediation capabilities.

There is a continuous demand for energy storage devices with high energy density in consumer electronics, electric vehicles, and the grid energy market. Although commercial lithium-ion batteries (LIBs) satisfy the current needs, the limited availability of raw materials and moderate specific charge capacities (SCC) of LIBS, motivated scientists to search for alternate anode materials for LIBs and also to find technologies beyond LIBs. In this work, we studied the potential of six Cobalt anti-MXenes (CoAs, CoB, CoP, CoS, CoSe, and CoSi), a class of newly discovered 2D materials, as anode materials for lithium, sodium, and potassium ion batteries (LIBs, NIBs, and KIBs). We found that these materials are good electrical conductors and have high adsorption stability for the alkali metal ions, which helps to prevent the formation of dendrites and increase the cycle life of the battery. They also show moderate to low migration energy barriers (MEBs), indicating the potential for faster charge-discharge kinetics. We also explained the slightly counter-intuitive result of observing low MEBs along with high adsorption stability. Furthermore, Co-anti-MXenes can adsorb multiple alkali atoms per formula unit, resulting in high specific charge capacities and low average anodic voltages. For example, as anode materials for lithium-ion batteries, CoP and CoSi have SCC values of 1075.4 mAh g-1 and 934 mAh g-1, and anodic voltages as low as 0.28 V and 0.43 V, respectively. Moreover, even the maximally metallated Co-anti-MXenes did not show agglomeration tendency at room temperature. Also, the volume expansion of these materials is minimum for both Li and Na adsorption. As a whole, we find that Co-anti-MXenes can act as promising anode materials for alkali metal ion batteries.

The current electric vehicle market is entirely dominated by lithium-ion batteries (LIBs). However, due to the limited and unequal distribution of LIB raw materials on earth, there is a continuous effort to design alternate storage devices. Among the alternatives to LIBs, sodium-ion batteries (NIBs) are at the forefront because sodium resources are ubiquitous worldwide and virtually inexhaustible. However, one of the major drawbacks of the NIBs is their low specific charge capacity. Since the specific charge capacity of a cell can be improved by increasing the specific charge capacity of the anode material, there is a constant effort to find suitable anode materials. Recent studies suggested that a cobalt-boride (CoB) anti-MXene material (a newly discovered two-dimensional material) can yield superior specific charge capacities for LIBs than traditional graphite-based anodes. Inspired by these findings, in this work, we considered six cobalt-based anti-MXene materials (Co-anti-MXenes), namely, CoAs, CoB, CoP, CoS, CoSe, and CoSi, and examined their competency as anode materials for NIBs. Our findings suggest that Co-anti-MXenes possess superior specific charge capacities (∼390–590 mA h g–1) than many well-studied anode materials such as MoS2 (146 mA h g–1), Cr2C (276 mA h g–1), expanded graphite (284 mA h g–1), and so forth. Moreover, their greater affinity (−0.55 to −1.16 eV) to Na atoms, along with reasonably small diffusion energy barriers (0.32–0.59 eV) and low-average sodiation voltages (0.2–0.64 V), suggests that these Co-anti-MXenes can serve as excellent anode materials for NIBs.

The current electric vehicle market is entirely dominated by lithium-ion batteries (LIBs). However, due to the limited and unequal distribution of LIB raw materials on earth, there is a continuous effort to design alternate storage devices. Among the alternatives to LIBs, sodium-ion batteries (NIBs) are at the forefront because sodium resources are ubiquitous worldwide and virtually inexhaustible. However, one of the major drawbacks of the NIBs is their low specific charge capacity. Since the specific charge capacity of a cell can be improved by increasing the specific charge capacity of the anode material, there is a constant effort to find suitable anode materials. Recent studies suggested that a cobalt-boride (CoB) anti-MXene material (a newly discovered two-dimensional material) can yield superior specific charge capacities for LIBs than traditional graphite-based anodes. Inspired by these findings, in this work, we considered six cobalt-based anti-MXene materials (Co-anti-MXenes), namely, CoAs, CoB, CoP, CoS, CoSe, and CoSi, and examined their competency as anode materials for NIBs. Our findings suggest that Co-anti-MXenes possess superior specific charge capacities (∼390–590 mA h g–1) than many well-studied anode materials such as MoS2 (146 mA h g–1), Cr2C (276 mA h g–1), expanded graphite (284 mA h g–1), and so forth. Moreover, their greater affinity (−0.55 to −1.16 eV) to Na atoms, along with reasonably small diffusion energy barriers (0.32–0.59 eV) and low-average sodiation voltages (0.2–0.64 V), suggests that these Co-anti-MXenes can serve as excellent anode materials for NIBs.

Cooperative supramolecular polymerization is important for the synthesis of functional supramolecular homo and block-copolymers of π-systems. Current strategies indicate the need for strong hydrogen bonding (H-bonding) and/or dipolar interactions in the π-systems to achieve cooperativity. In sharp contrast, here we report the cooperative supramolecular polymerization in alkyl chain substituted perylene diimides (alkyl PDIs) driven by dispersive interactions with molecular-level understanding. Moreover, alkyl PDIs follow a cooperative mechanism with cooperativity similar to the strong H-bonded π-systems (σ ∼10−5) despite the lack of strong H-bonding and dipolar interactions. Computer simulations show that this surprising phenomenon in alkyl PDIs is driven by the efficient dispersive interactions among the alkyl chains and π-cores due to their zigzag arrangement in the supramolecular polymer. Importantly, alkyl PDIs display cooperative supramolecular polymerization in both polar and non-polar solvents which is difficult for H-bonded/dipolar π-systems thus highlighting the advantages of dispersive interactions.

Per- and polyfluoroalkyl substances (PFASs) are synthetic contaminants found in drinking groundwater sources and a wide variety of consumer products. Because of their adverse environmental and human health effects, remediation of these persistent compounds has attracted significant recent attention. To shed mechanistic insight into their remediation, we present the first ab initio study of PFAS degradation via hydrated electrons - a configuration that has not been considered in previous studies up to this point. To capture these complex dynamical effects, we harness ab initio Molecular Dynamics (AIMD) simulations to probe the reactivities of perfluorooctanoic (PFOA) and perfluorooctane sulfonic acid (PFOS) with hydrated electrons in explicit water. We complement our AIMD calculations with advanced metadynamics sampling techniques to compute free energy profiles and detailed statistical analyses of PFOA/PFOS dynamics. While our calculations show that the activation barrier for C-F bond dissociation in PFOS is three times larger than PFOA, all the computed free energy barriers are still relatively low, resulting in a diffusion-limited process. We discuss our results in the context of recent studies on PFAS degradation with hydrated electrons to give insight into the most efficient remediation strategies for these contaminants. Most importantly, we show that the degradation of PFASs with hydrated electrons is markedly different than that of excess electrons/charges, a common (but largely incomplete) approach used in several earlier computational studies.

Enzymes are extremely complex catalytic structures with immense biological and technological importance. Nevertheless, their widespread environmental implementation faces several challenges, including high production costs, low operational stability, and intricate recovery and reusability. Therefore, the de novo design of minimalistic biomolecular nanomaterials that can efficiently mimic the biocatalytic function (bionanozymes) and overcome the limitations of natural enzymes is a critical goal in biomolecular engineering. Here, we report an exceptionally simple yet highly active and robust single amino acid bionanozyme that can catalyze the rapid oxidation of environmentally toxic phenolic contaminates and serves as an ultrasensitive tool to detect biologically important neurotransmitters similar to the laccase enzyme. While inspired by the laccase catalytic site, the substantially simpler copper-coordinated bionanozyme is ∼5400 times more cost-effective, four orders more efficient, and 36 times more sensitive compared to the natural protein. Furthermore, the designed mimic is stable under extreme conditions (pH, ionic strength, temperature, storage time), markedly reusable for several cycles, and displays broad substrate specificity. These findings hold great promise in developing efficient bionanozymes for analytical chemistry, environmental protection, and biotechnology.

This chapter focuses on the application of Density Functional Tight Binding (DFTB) to electronic-excited states, which has attracted significant attention for extending the computationally efficient approach to the time domain. The chapter highlights the use of real-time time-dependent-DFTB to probe the electron dynamics of large systems in external electric fields where the nuclei are held fixed. Surface hopping is a general mixed quantum-classical nonadiabatic dynamics methodology with many variants, such as fewest-switches surface-hopping, decoherence induced surface hopping, independent electron surface hopping, and others. The nuclei are propagated according to classical mechanics, and the forces on the nuclei, at any given instant of time, arise from a single adiabatic potential energy surfaces. The chapter illustrates the charge transfer dynamics of Phenyl-C61-butyric acid methyl ester/polythiophene, which is a model system for understanding photo-induced charge transfer dynamics in organic photovoltaics.

Four half-sandwich ruthenium(II) complexes were prepared by reacting dichloro(p-cymene)ruthenium(II) dimer or dichloro(benzene)ruthenium(II) dimer with 3,5-dimethylpyrazole (L1), bis(3,5-dimethylpyrazolyl)methane (L2), tris(3,5-dimethylpyrazolyl)methane (L3) and potassium tris(3,5-dimethylpyrazolyl)borate (L4) under air free conditions in good yields. All of the complexes were duly characterized by molar conductivity measurement, IR, UV, NMR spectroscopy, and elemental analysis. Solid state molecular structures of the complexes C2 and C4 were determined by single crystal X-ray crystallographic studies. Profound influence of the steric factors of the ligand substituents on the coordination chemistry and molecular structure of the complexes was profiled and compared with the related complexes in literature. An interesting solvent induced geometrical change was documented in the case of complex C4, bearing κ2-tris(3,5-dimethylpyrazolyl)borate and η6-p-cymene ligands. DFT calculations were used to understand the energetics and electronic structures of the polymorphic structures C4 and C4´.

Per- and polyfluoroalkyl substances (PFASs) are hazardous, carcinogenic, and bioaccumulative contaminants found in drinking water sources. To mitigate and remove these persistent pollutants, recent experimental efforts have focused on photo-induced processes to accelerate their degradation; however, the mechanistic details of these promising degradation processes remain unclear. To shed crucial insight on these electronic-excited state processes, we present the first study of photo-induced degradation of explicitly-solvated PFASs using excited-state, real-time time-dependent density functional theory (RT-TDDFT) calculations. Furthermore, our large-scale RT-TDDFT calculations show that these photo-induced excitations can be highly selective by enabling a charge-transfer process that only dissociates the Csingle bondF bond while keeping the surrounding water molecules intact. Collectively, the RT-TDDFT techniques used in this work (1) enable a new capability for probing photo-induced mechanisms that cannot be gleaned from conventional ground-state DFT calculations and (2) provide a rationale for understanding ongoing experiments that are actively exploring photo-induced degradation of PFASs and other environmental contaminants.

Borophene, the lightest among all Xenes, possesses extreme electronic mobility along with high carrier density and high Young's modulus. To accomplish device-quality borophene, novel approaches of realization of monolayers need to be urgently explored. In this work, micromechanical exfoliation is discovered to result in mono- and few-layered borophene of device quality. Borophene sheets are successfully fabricated down to monolayer thickness. Distinct crystallographic phases of borophene viz. XRD study reveals crystallographic phase transition from rhombohedral to several other eigen phases of borophene. The role of the destination substrates is held crucial in determining the final phase of the transferred sheet. The exfoliation energy is calculated by density functional theory. Molecular dynamics simulations are used to simulate the exfoliation process. Heterolayers of borophene, with black phosphorene (BP) or with molybdenum disulfide (MoS2) atomic sheets, are found to result in photoexcited coupling quantum states. Gold-coated borophene bestows promising anchoring capability for surface-enhanced Raman spectroscopy (SERS). Successful demonstration of the electronic behavior of micromechanically exfoliated borophene and excitonic behavior of borophene-based heterolayers will guide future generation devices not only in electronics and excitonics, but also in thermal management, electronic packaging, hydrogen storage, hybrid energy storage, and clean energy solutions.