Abstract Results of first-principles simulations of aqueous solution layers (with and without reactiveFeaq2+) adsorbed on the protonated (001) and (012) crystal surfaces of hematite (Fe2O3) are presented. PBE+U calculations show that the ground-state magnetic structures of both surfaces remain consistent with the bulk antiferromagnetic ordering and that the stable terminations are O-terminated. With these terminations, the calculated interlayer spacings in the bulk just beneath the surfaces agree with experiment to within 4.9%. AIMD simulations with spin order were performed using both PBE+U and SCAN gradient corrections. Results from the two methods are in semiquantitative agreement. At the hematite/water interfaces, the average OD···OA (donor oxygen to acceptor oxygen) distance for hydrogen bonds and the corresponding OD–H···OA angle are ∼2.8 Å and 158°, respectively, close to values in bulk water. The vibrational power spectrum indicates that interfacial water in our simulations exhibits a mixed liquid-like behavior and features of a more strongly hydrogen-bonded structure. The (001) surface structure is consistent with several CTR models, whereas for the (012) surface, the structure agrees with one of the CTR models but deviates from two others. The calculated distance between the first hydration layer and the outermost surface oxygen, however, is consistently larger than experimental estimates. This reveals significant discrepancies in the X-ray reflectivity measurements, likely due to unmodeled surface defects. For the (001) surface, the first-layer height is 2.59 Å by PBE+U and 2.51 Å by SCAN, compared with an experimental value of 1.9 ± 0.6 Å. For the (012) surface, a single adsorbed water layer at 2.34 Å is found, whereas experimental studies report two absorbed water layers with the first located at ∼1.0 Å and the second between 1.4 Å and 2.6 Å from the surface. AIMD simulations of Fe2+ adsorption reveal distinct site preferences at the two hematite interfaces. On the (001) surface, the adsorbed Fe2+ achieves 6-fold coordination by forming three bonds with surface hydroxyl groups and three with water molecules, whereas on the (012) surface it bonds to only two hydroxyls and four water molecules. Projected density of states (PDOS) analysis shows that the energy gap between Fe2+ states and surface oxygen bands is ∼0.5 eV on the (001) surface but ∼1.5 eV on the (012) surface. The greater delocalization of the HOMO into the (001) surface, together with these PDOS features, suggests that absorption and electron transfer are more favorable on (001) than on (012), consistent with their surface magnetic structures.

We have performed atom-specific vibrational analyses of a large number of perfluorooctanoic acid isomers as well as their anions and show that differences in the vibrational features of corresponding anion and neutral molecules clearly identify the C-F bonds that are most strongly activated in the anions. We discuss two analysis tools for associating vibrational modes with individual atoms, both based on density functional theory calculations. The first involves computing the Einstein frequencies for a given atom and the second projects the full vibrational spectrum onto a given atom using participation factors derived from the normal mode eigenvectors. We show that the two methods give results that are qualitatively the same and that either can be used to identify the most labile C-F bonds in the anions. We also show that the results of the vibrational analyses are consistent with systematically computed F atom removal energies. The vibrational analysis tools are shown to be related to the local mode analysis of Cremer and co-workers. Finally, we compare and contrast branched and linear PFAS molecules and analogous hydrogenated carbon chains.

We assess the performance of the Quantum Flow (QFlow) algorithm employing cost-effective solvers based on the unitary coupled-cluster ansatz with single and double excitations (QFlow-SD). The resulting energies are benchmarked against those obtained with an analogous QFlow formulation defined in the same active spaces but augmented by higher-rank excitations, including triples and quadruples (QFlow-SDTQ). Across all molecular systems considered, QFlow-SD exhibits close agreement with results from the canonical unitary coupled cluster with singles and doubles framework, while requiring substantially fewer qubits than the latter. For the water molecule in the cc-pVTZ basis, we further demonstrate the performance of a composite two-step downfolding strategy. In this approach, an initial coupled-cluster downfolding based on the double unitary coupled-cluster ansatz is followed by a QFlow treatment within the resulting target space, illustrating the effectiveness of combining classical downfolding with quantum flow optimization.

Iron oxide minerals regulate the flux of electrons in the environment and are important hosts for trace and minor, yet critical, elements. Here, we present the first evidence of a direct link between the local coordination environments of Ni and Zn and the redox properties of their host phase goethite (α-FeOOH), the most abundant Fe(III) (oxyhydr)oxide at Earth's surface. We used aqueous redox measurements to show that the redox potential EH0, and hence the mineral's stability, follows the order: pure goethite ≥ Zn-goethite > Ni-goethite. Parallel X-ray absorption and scattering measurements demonstrate, using quantum-informed analysis, that the local coordination environment of the smaller impurity, Ni, causes more bulk strain energy than Zn, which nearly accounts for the difference in EH0 between Ni- and Zn-goethite. Our theory-informed, experimental study reveals how two common impurities affect the stability of goethite with implications for the biogeochemical reactivity of Fe(III) (oxyhydr)oxide in mediating elemental and electron fluxes in the environment.

Up to 20% of rare earth elements (REEs) in ion adsorption deposits (IADs) are associated with iron oxide minerals, primarily goethite. Often termed "non-extractable", goethite-hosted REEs are thought to be structurally incorporated into the mineral lattice. The large mismatch in size and charge density between REEs (ionic radius, r = 0.86-1.03 Å) and Fe3+ (r = 0.65 Å), however, makes direct substitution energetically unfavorable. To determine REE compatibility with and incorporation into goethite on the atomic level, we used X-ray pair distribution function analysis (PDF) and LIII-edge extended X-ray absorption fine structure (EXAFS) spectroscopy. The compatibility of REEs with goethite and the precursor ferrihydrite (FH) is Lu ≥ Yb ≫ Dy > Nd for both phases. Nd and Dy primarily form secondary amorphous phases, with <30% Nd and Dy incorporated into goethite. In the FH precursor at pH 6.8, Yb and Lu assumed a local REE-OOH like structure with next nearest neighbor Fe. The Yb, Lu, and Nd-FH samples were also matured at ambient conditions for 100 days; despite the presence of only ∼5% goethite, Lu and Yb were 42% and 100% in goethite-like structural environments, respectively, whereas the PDF and EXAFS of Nd showed little evidence of any incorporation. Using ab initio molecular dynamics (AIMD) to model the EXAFS, we determined the presence of protonated Fe vacancies, edge-sharing with structural Lu and Yb, likely helped accommodate these REEs in the goethite structure. Incorporation into Fe oxyhydroxides thus potentially fractionates the REEs during weathering associated with formation of lateritic and ion adsorption deposits.

The transformative impact of modern computational paradigms and technologies, such as high-performance computing (HPC), quantum computing, and cloud computing, has opened up profound new opportunities for scientific simulations. Scalable computational chemistry is one beneficiary of this technological progress. The main focus of this paper is on the performance of various quantum chemical formulations, ranging from low-order methods to high-accuracy approaches, implemented in different computational chemistry packages and libraries, such as NWChem, NWChemEx, Scalable Predictive Methods for Excitations and Correlated Phenomena, ExaChem, and Fermi-Löwdin orbital self-interaction correction on Azure Quantum Elements, Microsoft's cloud services platform for scientific discovery. We pay particular attention to the intricate workflows for performing complex chemistry simulations, associated data curation, and mechanisms for accuracy assessment, which is demonstrated with the Arrows automated workflow for high throughput simulations. Finally, we provide a perspective on the role of cloud computing in supporting the mission of leadership computational facilities.

Reactions at mineral interfaces with aqueous solutions control many geochemical and biogeochemical processes in the Earth's critical zone. At the molecular level, insights into important properties such as the structure of the electrical double layer (EDL) at specific mineral interfaces continue to improve due to the increasing fidelity of laboratory instrumentation and computational approaches. However, molecular simulation approaches suffer from limited reach into relevant scales of time, length, and system complexity. To span this gap, a novel hybrid approach that couples first-principles plane-wave density functional theory (DFT) with classical DFT (cDFT) is demonstrated and applied to calcite (104) interfaces with various electrolytes. In this approach, a region of interest described by using DFT interacts with the surrounding medium described by using cDFT to arrive at a self-consistent ground state. Benchmarking against experimental observations and entirely first-principles DFT simulations demonstrates that this hybrid model efficiently encompasses the key short-range and collective interactions in the EDL. Simulations of calcite (104)/solution interfaces reveal the key static and dynamic polarization interactions that give rise to structuring of ions and water. Ion hydration interactions have the strongest effect on the depth of the first minimum in the density distribution of counterions at the surface, and the position and width of the first density peak are largely determined by the strength of ion-correlation forces. Finer details of ion distributions are controlled by the mutual polarization of the calcite surface and interfacial electrolyte. This new ability to efficiently and rigorously predict EDL structure at mineral surfaces in contact with complex solutions paves the way to accurately modeling sorption, nucleation, dissolution, and growth in realistic systems.

Understanding the reactivity of metal cations with various reaction gases in inductively coupled plasma tandem mass spectrometry (ICP-MS/MS) is important to determine the best gas to use for a given analyte/interference pair. In this study, nitric oxide (NO) was investigated as the reaction gas following previous experimental designs. The reactions with 50 elements were investigated to examine periodic trends in reactivity, validate theoretical modeling of reaction enthalpies as a method to screen reactant gases, and provide a baseline data set for potential in-line gas separation methods. ICP-MS/MS studies involving actinides are typically limited to Th, U, and Pu, with analyses of Np and Am rarely reported in the literature. To date, only two previous methods have investigated the use of NO in ICP-MS/MS analyses. To showcase the utility of NO, a method was developed to measure Pu-239 in the presence of environmental matrix constituent and other actinides, like what could be expected from postdetonation debris, with no chemical separation prior to analysis. Pu-239(+) was reacted to form (PuO+)-Pu-239-O-16, eliminating interferences derived from the sample matrix by measuring the Pu-239(+) intensity at m/z = 255 ((PuO+)-Pu-239-O-16). To validate NO for (UH+)-U-238-H-1 interference removal in environmental matrices, standard reference materials were diluted to 1 mg/g of solution and spiked to 0.05 pg/g of Pu-239 and 1 mu g/g U-238 (Pu/U = 5 x 10(-8)). Measured Pu-239 concentrations were within 6% of the spiked value. These results demonstrate that reliable Pu-239 measurements can be made at levels relevant to nuclear forensics without the need for extensive chemical matrix separation prior to analysis.

This study investigated the reaction pathway of 2,4-dinitroanisole (DNAN) on the pyrogenic carbonaceous matter (PCM) to assess the scope and mechanism of PCM-facilitated surface hydrolysis. DNAN degradation was observed at pH 11.5 and 25 degrees C with a model PCM, graphite, whereas no significant decay occurred without graphite. Experiments were performed at pH 11.5 due to the lack of DNAN decay at pH below 11.0, which was consistent with previous studies. Graphite exhibited a 1.78-fold enhancement toward DNAN decay at 65 degrees C and pH 11.5 relative to homogeneous solution by lowering the activation energy for DNAN hydrolysis by 54.3 +/- 3.9%. This is supported by our results from the computational modeling using Car-Parrinello simulations by ab initio molecular dynamics/molecular mechanics (AIMD/MM) and DFT free energy simulations, which suggest that PCM effectively lowered the reaction barriers by approximately 8 kcal mol(-1) compared to a homogeneous solution. Quaternary ammonium (QA)-modified activated carbon performed the best among several PCMs by reducing DNAN half-life from 185 to 2.5 days at pH 11.5 and 25 degrees C while maintaining its reactivity over 10 consecutive additions of DNAN. We propose that PCM can affect the thermodynamics and kinetics of hydrolysis reactions by confining the reaction species near PCM surfaces, thus making them less accessible to solvent molecules and creating an environment with a weaker dielectric constant that favors nucleophilic substitution reactions. Nitrite formation during DNAN decay confirmed a denitration pathway, whereas demethylation, the preferred pathway in homogeneous solution, produces 2,4-dinitrophenol (DNP). Denitration catalyzed by PCM is advantageous to demethylation because nitrite is less toxic than DNAN and DNP. These findings provide critical insights for reactive adsorbent design that has broad implications for catalyst design and pollutant abatement.

Many important geochemical and biogeochemical processes involve reactivity and dynamics in complex solutions. Gaining a fundamental understanding of these reaction mechanisms is a challenging goal that requires advanced computational and experimental approaches. However, important techniques such as molecular simulation have limitations in terms of scales of time, length and system complexity. Furthermore, among currently available solvation models, there are very few designed to describe the interaction between the molecular scale and mesoscale. To help address this challenge, here we establish a novel hybrid approach that couples first principle plane-wave density functional theory (DFT) with classical density functional theory (cDFT). In this approach, a region of interest described by ab initio molecular dynamics (AIMD) interacts with the surrounding medium described using cDFT to arrive at a self-consistent ground state. cDFT is a robust but efficient mesoscopic approach to accurate thermodynamics of bulk electrolyte solutions over a wide concentration range (up to 2 molar concentrations). Benchmarking against commonly used continuum models of solvation such as SMD, as well as experiment, demonstrates that our hybrid AIMD/cDFT method is able to produce reasonable solvation energies for a variety of molecules and ions. With this model, we also examined solvent effects on a prototype S$_N$2 reaction of the nucleophilic attack of a chloride ion on methyl chloride in solution. The resulting reaction pathway profile and the solution phase barrier agree well with the experiment, showing that our AIMD/cDFT hybrid approach can provide insight into the specific role of solvent on the reaction coordinate.

Modern plane-wave DFT methods and software (contained in the NWChem and NWChemEx packages) that allow for both geometry optimization and ab initio molecular dynamics simulations (AIMD) are described. Significant emphasis is placed on aspects of these methods that are of interest to computational chemists and useful for simulating chemistry, including techniques for calculating charged systems, exact exchange (i.e., hybrid DFT methods), and highly efficient AIMD/MM methods. Sample applications for the hydrolysis of nitroaromatic molecules, the structure of the goethite+water interface, and AIMD-EXAFS calculations for uranium metal impurities in iron-(oxyhydr)oxides are described.

Carbonyl sulfide (OCS) was used as a reaction gas to investigate gas phase metal (M+) ion-molecule reactions using the Agilent 8900 inductively coupled plasma tandem mass spectrometer (ICP-MS/MS) to yield insight on how this gas may be used to remove isobaric interferences in analytical measurements. The experimental work was paired with density functional theory (DFT) calculations of the reaction enthalpy to predict whether M+ will react with OCS. A multi-element standard containing 46 elements ranging from 9 to 208 u was analyzed in the presence and absence of OCS. When a reaction was observed, the dominant product was the sulfide (MS+). Oxide products were also observed for many M+ but formation was less efficient with OCS than previously observed with other reaction gases. This is likely due to the weaker OC-S bond that makes MS+ formation more favorable. Increasing the flow rate from 0.1 to 0.2 mL/min (corresponding to a change in reaction gas pressure from 0.35 to 0.53 Pa (2.6 to 4.0 mTorr)) generally resulted in greater MS+ production, including the secondary product MS2+ for a few cations. The early lanthanide series ions (La+, Ce+, Pr+ and Nd+) produced greater quantities of MO+ at the higher pressure, although MS+ products were still the dominant product. The DFT-predicted reaction enthalpies were consistent with the observed sulfide formation, with an accuracy >90%; however, model predictions were less accurate for the minor and higher order products (< 77% for MO+). The work presented here continues a systematic study of ion-molecule reactions in ICP-MS/MS to understand and develop new and novel ways to analyze complex mixtures with minimal pre-analysis treatment.

A procedure for defining virtual spaces, and the periodic one-electron and two-electron integrals, for plane-wave second quantized Hamiltonians has been developed and demonstrated using full configuration interaction (FCI) simulations and variational quantum eigensolver (VQE) circuits on Quantinuum's ion trap quantum computers accessed through Microsoft's Azure Quantum service. This work is an extension to periodic systems of a new class of algorithms in which the virtual spaces were generated by optimizing orbitals from small pairwise CI Hamiltonians, which we term as correlation optimized virtual orbitals with the abbreviation COVOs. In this extension, the integration of the first Brillouin zone is automatically incorporated into the two-electron integrals. With these procedures we have been able to derive virtual spaces, containing only a few orbitals, that were able to capture a significant amount of correlation. The focus in this manuscript is on comparing the simulations of small molecules calculated with plane-wave basis sets with large periodic unit cells at the Γ-point, including images, to results for plane-wave basis sets with aperiodic unit cells. The results for this approach were promising as we were able to obtain good agreement between periodic and aperiodic results for an LiH molecule. Simulations performed on the Quantinuum H1-1 quantum computer were able to produce surprisingly good energies, reproducing the FCI values for the 1 COVO Hamiltonian to within 11 milliHartree (6.9 kcal/mol), when corrected for noise.

Chloroform (CF) is a widely used chemical reagent and disinfectant and a probable human carcinogen. The extensive literature on halocarbon reduction with zerovalent iron (ZVI) shows that transformation of CF is slow, even with nano, bimetallic, sulfidated, and other modified forms of ZVI. In this study, an alternative method of ZVI modification─involving simultaneous sulfidation and nitridation through mechanochemical ball milling─was developed and shown to give improved degradation of CF (i.e., higher degradation rate and inhibited H2 evolution reaction). The composite material (denoted as S-N(C)-ZVI) gave synergistic effects of nitridation and sulfidation on CF degradation. A complete chemical reaction network (CRN) analysis of CF degradation suggests that O-nucleophile-mediated transformation pathways may be the main route for the formation of the terminal nonchlorinated products (formate, CO, and glycolic polymers) that have been used to explain the undetected products needed for mass balance. Material characterizations of the ZVI recovered after batch experiments showed that sulfidation and nitridation promoted the formation of Fe3O4 on the S-N(C)-ZVI particles, and the effect of aging on CF degradation rates was minor for S-N(C)-ZVI. The synergistic benefits of sulfidation and nitridation on CF degradation were also observed in experiments performed with groundwater.

This paper summarizes developments in the NWChem computationalchemistry suite since the last major release (NWChem 7.0.0). Specifically,we focus on functionality, along with input blocks, that is accessiblein the current stable release (NWChem 7.2.0) and in the "master"development branch, interfaces to quantum computing simulators, interfacesto external libraries, the NWChem github repository, and containerizationof NWChem executable images. Some ongoing developments that will beavailable in the near future are also discussed.

Carbon dioxide (CO2) was used as a reaction gas to investigate the gas‐phase ion‐molecule interactions using the Agilent 8900 ICP‐MS/MS. A solution containing forty‐five elements representative of the periodic table was used to supply the ions to react with CO2 in the collision/reaction cell (CRC). The only significant product ions formed were monoxides. The general reactivity was shown to be consistent with density functional theory (DFT)‐predicted reaction enthalpies, such that all predicted exothermic reactions produced product ions at levels of at least 1% of the unreacted ion. Most endothermic reactions observed had sufficient kinetic energy in excess of the reaction enthalpies. Our results suggest that reaction enthalpy is a reasonable predictor of reactivity with CO2 on the timescales of the interactions in non‐thermal ICP‐MS/MS systems. The ease and rapidity of data collection with the ICP‐MS/MS and DFT calculations using the NWChem suite has value given the scarcity of thermochemical data of CO2 reactions in the literature. These studies are especially useful for the identification of targeted reaction chemistries to be leveraged for analytical method development, such as for the inline separation of isobaric interferences from analytes of interest.

Iron (oxyhydr)oxides are common soil minerals that can be host phases to a large number of environmentally important elements, such as the micronutrient Zn. The structure and hydration level of these host phases change significantly if the soil is heated, as is the case during wildfires, for example. Steady increases in the frequency and scale of wildfires in recent decades thus raises the question of whether these events significantly impact the geochemical availability of trace elements incorporated within the reacting iron (oxyhydr)oxides. In this contribution, we focus on Zn-substitution into two abundant iron minerals in soil, hematite (alpha-Fe2O3) and goethite (alpha-FeOOH). Using ab initio molecular dynamics (ALMD)-informed extended X-ray absorption fine structure (EXAFS) spectra, we determine the local environment of Zn in hydrothermally precipitated material and after annealing at 500 and 800 degrees C. Results show that Zn was incorporated into the hematite structure by preferential formation of coupled O-Fe and protonated Fe vacancies (P-Fe). These defects increase in density around Zn upon heating the samples to 500 degrees C, particularly P-Fe. Heating to higher temperatures produced franklinite (ZnFe2O4), a highly refractory zinc iron spinet. This difference in structure and hydration of Zn sites suggests soil mineralogy and wildfire temperature impact the local coordination of trace elements, which may play a role in controlling the bioavailability of these elements.