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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 1,261 records · Page 70
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Light controlled reversible Michael addition of cysteine: a new tool for dynamic site-specific labeling of proteins

Cysteine-based Michael addition is a widely employed strategy for covalent conjugation of proteins, peptides, and drugs. The covalent reaction is irreversible in most cases, leading to a lack of control over the process. Utilizing spectroscopic analyses along with X-ray crystallographic studies, we demonstrate Michael addition of an engineered cysteine residue in human Cellular Retinol Binding Protein II (hCRBPII) with a coumarin analog that creates a non-fluorescent complex. UV-illumination reverses the conjugation, yielding a fluorescent species, presumably through a retro-Michael process. Furthermore, this series of events can be repeated between a bound and non-bound form of the cysteine reversibly, resulting in the ON–OFF control of fluorescence. The details of the mechanism of photoswitching was illuminated by recapitulation of the process in light irradiated single crystals, confirming the mechanism at atomic resolution.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

BODIPY and dipyrrin as unexpected robust anchoring groups on TiO 2 nanoparticles

Covalent attachment of molecules to metal oxide surfaces typically demands the presence of an anchoring group that in turn requires synthetic steps to introduce. BODIPY (4,4-difluoro-4-bora-3a,4a-diaza-s-indacene) chromophores have long been used in dye-sensitized solar cells, but carboxylic acid groups typically had to be installed to act as surface anchors. We now find that even without the introduction of such anchors, the unmodified BODIPY can bind to TiO 2 surfaces via its BF 2 group through boron–oxygen surface bonds. Dipyrrin, the parent molecule of BODIPY, is also capable of binding directly to TiO 2 surfaces, likely through its chelating nitrogen atoms. These binding modes prove to be even more robust than that of an installed carboxylate and offer a new way to attach molecular complexes to surfaces for (photo)catalytic applications since, once bound, we show that surface bound BODIPY and dipyrrin derivatives exhibit ultrafast photoinjection of electrons into the conduction band of TiO 2 .

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Voltage-dependent excitation dynamics in UV-absorbing organic photovoltaics with efficient charge transfer exciton emission

Intermolecular charge-transfer excitons play a central role in determining the performance of organic solar cells as their voltage-dependent formation, dissociation, and recombination dynamics contribute to photocurrent generation, radiative/nonradiative voltage losses, and photovoltaic fill factor. Here, we explore the properties of brightly-emitting wide energy gap (>2 eV) charge transfer excitons by measuring the voltage-dependent photoluminescence, photocurrent, and ultrafast pump–probe transient absorption spectra of organic solar cells employing five UV-absorbing donor molecules that differ only by the length of the oligophenylene or acene group at their core. We find that organic solar cells with a strong correlation between their voltage-dependent photocurrent and charge-transfer exciton photoluminescence have low photovoltaic fill factors as they require voltage to facilitate efficient charge-transfer exciton dissociation. In contrast, solar cells that are efficient can readily generate charges without an applied field and have a separate population of tightly-bound charge-transfer excitons that are responsible for emission. Furthermore, considering that the sum of all excitation loss rates (i.e., recombination and charge extraction) must be equal to the excitation generation rate in the steady state, these voltage-dependent data allow us to solve for the voltage-dependent fate of all excitations in the solar cells and estimate upper and lower bounds for geminate and non-geminate recombination, respectively.

14 SOLAR ENERGY↗
DOE OSTI2022

Historical record of tritium from tree cores at the Savannah River Site

In this work, tree cores from various locations at the Savannah River Site (SRS) and local area were measured for total tritium (T) content and T speciation to include tritiated water (HTO), exchangeable organic bound T (E-OBT) and non-exchangeable organic bound T (NE-OBT) species. The tree cores dated back to the 1960's or prior which provided an opportunity to measure T over the last 60–70 years. The total T levels from pine and oak tree cores were consistent with the record of known T atmospheric releases from nuclear activities at the SRS between the mid-1950's and 1990's with a notable peak in T tree core levels during the late 1960's. The T speciation data for some tree core samples from SRS demonstrated elevated levels of OBT : HTO and NE-OBT : E-OBT identified primarily in the last ~20 years due to T inputs from remedial irrigation with OBT-rich pond water. Elevated but lower levels of OBT : HTO and NE-OBT : E-OBT were observed due to inputs from SRS operations during the last 70 years and prior to irrigation with the T-rich pond water.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2023

A conformational equilibrium in the nitrogenase MoFe protein with an α-V70I amino acid substitution illuminates the mechanism of H 2 formation

Study of α-V70I-substituted nitrogenase MoFe protein identified Fe6 of FeMo-cofactor (Fe 7 S 9 MoC-homocitrate) as a critical N 2 binding/reduction site. Freeze-trapping this enzyme during Ar turnover captured the key catalytic intermediate in high occupancy, denoted E 4 (4H), which has accumulated 4[e — /H + ] as two bridging hydrides, Fe2–H–Fe6 and Fe3–H–Fe7, and protons bound to two sulfurs. E 4 (4H) is poised to bind/reduce N 2 as driven by mechanistically-coupled H 2 reductive-elimination of the hydrides. This process must compete with ongoing hydride protonation (HP), which releases H 2 as the enzyme relaxes to state E 2 (2H), containing 2[e — /H + ] as a hydride and sulfur-bound proton; accumulation of E 4 (4H) in α-V70I is enhanced by HP suppression. EPR and 95 Mo ENDOR spectroscopies now show that resting-state α-V70I enzyme exists in two conformational states, both in solution and as crystallized, one with wild type (WT)-like FeMo-co and one with perturbed FeMo-co. These reflect two conformations of the Ile residue, as visualized in a reanalysis of the X-ray diffraction data of α-V70I and confirmed by computations. EPR measurements show delivery of 2[e — /H + ] to the E 0 state of the WT MoFe protein and to both α-V70I conformations generating E 2 (2H) that contains the Fe3–H–Fe7 bridging hydride; accumulation of another 2[e — /H + ] generates E 4 (4H) with Fe2–H–Fe6 as the second hydride. E 4 (4H) in WT enzyme and a minority α-V70I E 4 (4H) conformation as visualized by QM/MM computations relax to resting-state through two HP steps that reverse the formation process: HP of Fe2–H–Fe6 followed by slower HP of Fe3–H–Fe7, which leads to transient accumulation of E 2 (2H) containing Fe3–H–Fe7. In the dominant α-V70I E 4 (4H) conformation, HP of Fe2–H–Fe6 is passively suppressed by the positioning of the Ile sidechain; slow HP of Fe3–H–Fe7 occurs first and the resulting E 2 (2H) contains Fe2–H–Fe6. Importantly, it is this HP suppression in E 4 (4H) that enables α-V70I MoFe to accumulate E 4 (4H) in high occupancy. In addition, HP suppression in α-V70I E 4 (4H) kinetically unmasks hydride reductive-elimination without N 2 -binding, a process that is precluded in WT enzyme.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Influence of tritium exposure route on vegetation types at the Savannah River Site

Plant, soil, water, and other media from various locations at the Savannah River Site were measured for total tritium (T) content and T speciation to characterize T in these areas, as well as investigate its uptake behavior and the transport of T species in these media. This characterization included the isolation and measurement of T in tritiated water (HTO), and (when possible) exchangeable organic bound T (E-OBT) and non-exchangeable organic bound T (NE-OBT). Two areas of interest were investigated: (1) a holding pond with T-contaminated water and (2) open basins or streams with low to background levels of T. Water in the holding pond is used to irrigate forest plots in the local area as a T remediation approach. This study compares the analytical data for water, soil/sediment, plants, and lichens from these locations. Here, the results indicate that the behavior of T in plants from these areas can be a function of one or more of the following: seasonal precipitation, the plant's primary route of access to the T-contamination (such as water uptake through the root vs. shoot), plant physical location (relative to T-contaminated water sources), plant rooting depth, pond water level, and plant height above the ground. Total T concentrations were lowest in the un-irrigated forest plants, followed by irrigated forest plants, shallow rooting plants near the pond, deep rooting plants further from the pond, and then water-saturated plants. The OBT:HTO and NE-OBT:E-OBT ratios were always greater for plants from irrigated forest plots compared to those from the holding pond.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Acidities of MgO surface sites: implications for the formation mechanism of Mg(OH) 2

The hydroxylation of periclase (MgO) to brucite (Mg(OH) 2 ) is thought to be an important intermediate step when using MgO to capture CO 2 from the atmosphere. However, the mechanism of hydroxylation of MgO to form Mg(OH) 2 is poorly understood. In this work, we used atomic-scale density functional tight binding simulations coupled with the metadynamics rare event method to analyze the surface chemistry of MgO and the acid dissociation equilibrium constants (pK a ) of its surface sites. The method and parameters were validated by calculating the pK a for hydroxylation of the first shell water bound to aqueous Mg 2+ ion. The pK a value derived using a probabilistic method was 12.3, which is in fair agreement with the accepted value of 11.4, with the difference between them equal to a ∼5 kJ mol −1 error in the calculations. We then extended these pK a calculations to probe the hydroxylation reactions of the surface sites of the MgO(100)–water interface, arriving at pK a s of 5.4 to deprotonate terminal water molecules bound to the surface magnesium sites (η-OH 2 or 〉MgOH 2 ), and 13.9 to deprotonate hydroxylated bridging oxygen sites (μ 5 -oxo or 〉O). Hydroxide (OH − ) adsorption on the surface was also probed and found to be less thermodynamically favorable than deprotonation of the terminal water molecule. The plausibility of the computed pK a s was verified using an activity-based speciation model and compared to pH measurements of water equilibrated with MgO nanoparticles and single crystals. The model predicted a solution pH of 7.1 when surface sites buffered and the pH of 12.0 when MgO dissolution dominated. These are close to the experimental initial solution pHs of 7–7.5 and the long term pHs of ∼10.5. The similarity suggests that the calculated pK a values from the DFTB+/metadynamics simulations are plausible and that these methods can be a useful tool to probe reaction mechanisms involving covalent bonds.

Adapa, Sai Krishna Reddy [Oak Ridge National Labor↗
DOE OSTI2025

Testing the thermal Sunyaev-Zel’dovich power spectrum of a halo model using hydrodynamical simulations

Statistical properties of large-scale cosmological structures serve as powerful tools for constraining the cosmological properties of our Universe. Tracing the gas pressure, the thermal Sunyaev-Zel’dovich (tSZ) effect is a biased probe of mass distribution and, hence, can be used to test the physics of feedback or cosmological models. Therefore, it is crucial to develop robust modelling of hot gas pressure for applications to tSZ surveys. Since gas collapses into bound structures, it is expected that most of the tSZ signal is within halos produced by cosmic accretion shocks. Hence, simple empirical halo models can be used to predict the tSZ power spectra. In this study, we employed the HMx halo model to compare the tSZ power spectra with those of several hydrodynamical simulations: the Horizon suite and the Magneticum simulation. We examine various contributions to the tSZ power spectrum across different redshifts, including the one- and two-halo term decomposition, the amount of bound gas, the importance of different masses, and the electron pressure profiles. Our comparison of the tSZ power spectrum reveals discrepancies between the halo model and cosmological simulations that increase with redshift. We find a 20% to 50% difference between the measured and predicted tSZ angular power spectrum over the multipole range ℓ = 10 3 − 10 4 . Our analysis reveals that these differences are driven by the excess of power in the predicted two-halo term at low k and in the one-halo term at high k . At higher redshifts ( z ∼ 3), simulations indicate that more power comes from outside the virial radius than from inside, suggesting a limitation in the applicability of the halo model. We also observe differences in the pressure profiles, despite the fair level of agreement on the tSZ power spectrum at low redshift with the default calibration of the halo model. In conclusion, our study suggests that the properties of the halo model need to be carefully controlled against real or mock data to be proven useful for cosmological purposes.

Ayçoberry, Emma (ORCID:0000000292351195)↗
DOE OSTI2020

Re-evaluation of the TSL for Yttrium Hydride

Yttrium hydride (YH x ) is of interest as a high-temperature moderator material in advanced nuclear reactor systems because of its superior ability to retain hydrogen at elevated temperatures. Thermal neutron scattering laws (TSL) for hydrogen bound in yttrium hydride (H-YH 2 ) and yttrium bound in yttrium hydride (Y-YH 2 ) were previously evaluated by Naval Nuclear Laboratory using the ab initio approach and released in ENDF/B-VIII.0. In that work, density functional theory, incorporating the generalized gradient approximation (GGA) for the exchange-correlation energy, was used to simulate the face-centered cubic structure of YH 2 and calculate the interatomic Hellmann-Feynman forces for a 2×2×2 supercell containing 96 atoms. Lattice dynamics calculations using PHONON were used to determine the phonon density of states. The calculated phonon density of states for H and Y in YH 2 were then used to prepare H-YH 2 and Y-YH 2 TSL evaluations, in the incoherent approximation, using the LEAPR module of NJOY. In addition, elastic scattering was assumed to be incoherent for both H and Y. While the incoherent elastic scattering approximation is appropriate for H-YH 2 , it introduces an undesirable approximation for Y-YH 2 . In this work, we re-evaluate the TSL for Y-YH 2 using FLASSH (Full Law Analysis Scattering System Hub). Y-YH 2 is evaluated using the FLASSH generalized coherent elastic scattering capability in order to capture the Bragg peaks associated with the YH 2 crystal structure which were neglected in the prior NJOY-based evaluation due to limitations in LEAPR. An experimental approach to validate the Y-YH 2 TSL using neutron transmission measurements is discussed.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2021

Probabilistic error estimation for non-intrusive reduced models learned from data of systems governed by linear parabolic partial differential equations

This work derives a residual-based a posteriori error estimator for reduced models learned with non-intrusive model reduction from data of high-dimensional systems governed by linear parabolic partial differential equations with control inputs. It is shown that quantities that are necessary for the error estimator can be either obtained exactly as the solutions of least-squares problems in a non-intrusive way from data such as initial conditions, control inputs, and high-dimensional solution trajectories or bounded in a probabilistic sense. Here, the computational procedure follows an offline/online decomposition. In the offline (training) phase, the high-dimensional system is judiciously solved in a black-box fashion to generate data and to set up the error estimator. In the online phase, the estimator is used to bound the error of the reduced-model predictions for new initial conditions and new control inputs without recourse to the high-dimensional system. Numerical results demonstrate the workflow of the proposed approach from data to reduced models to certified predictions.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2020

Clostridioides difficile Toxin A Remodels Membranes and Mediates DNA Entry Into Cells to Activate Toll-Like Receptor 9 Signaling

Background & Aims: Clostridioides difficile toxin A (TcdA) activates the innate immune response. TcdA co-purifies with DNA. Toll-like receptor 9 (TLR9) recognizes bacterial DNA to initiate inflammation. We investigated whether DNA bound to TcdA activates an inflammatory response in murine models of C difficile infection via activation of TLR9. Methods: We performed studies with human colonocytes and monocytes and macrophages from wild-type and TLR9 knockout mice incubated with TcdA or its antagonist (ODN TTAGGG) or transduced with vectors encoding TLR9 or small-interfering RNAs. Cytokine production was measured with enzyme-linked immunosorbent assay. We studied a transduction domain of TcdA (TcdA 57-80 ), which was predicted by machine learning to have cell-penetrating activity and confirmed by synchrotron small-angle X-ray scattering. Intestines of CD1 mice, C57BL6J mice, and mice that express a form of TLR9 that is not activated by CpG DNA were injected with TcdA, TLR9 antagonist, or both. Enterotoxicity was estimated based on loop weight to length ratios. A TLR9 antagonist was tested in mice infected with C difficile. We incubated human colon explants with an antagonist of TLR9 and measured TcdA-induced production of cytokines. Results: The TcdA 57-80 protein transduction domain had membrane remodeling activity that allowed TcdA to enter endosomes. TcdA-bound DNA entered human colonocytes. TLR9 was required for production of cytokines by cultured cells and in human colon explants incubated with TcdA. TLR9 was required in TcdA-induced mice intestinal secretions and in the survival of mice infected by C difficile. Even in a protease-rich environment, in which only fragments of TcdA exist, the TcdA 57-80 domain organized DNA into a geometrically ordered structure that activated TLR9. Conclusions: TcdA from C difficile can bind and organize bacterial DNA to activate TLR9. TcdA and TcdA fragments remodel membranes, which allows them to access endosomes and present bacterial DNA to and activate TLR9. Rather than inactivating the ability of DNA to bind TLR9, TcdA appears to chaperone and organize DNA into an inflammatory, spatially periodic structure.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2024

On the Impact of Bus Dwelling on Macroscopic Fundamental Diagrams

Network macroscopic fundamental diagrams (MFDs) have recently been shown to exist in real-world urban traffic networks. When present, MFDs can be used to model traffic dynamics within an urban network by dividing the network into a set of spatially compact homogeneous regions and tracking the average level of congestion in each region. Existing analytical methods to estimate MFD mostly focus on the behavior of a single type of vehicle and do not capture the patterns of mixed traffic (e.g., cars and buses). The existence of buses matters since a bus will block the movements of other vehicles when it dwells at the bus stop. This paper proposes an analytical method to estimate the impact of bus dwelling on a network’s MFD based on the network’s geometric features, traffic control strategies, and bus operation parameters, and validates the performance of the proposed method using simulations based on microscopic traffic models. Comparisons of the analytical and simulation results show that the proposed analytical method can generally provide a good estimate of the lower bound and upper bound of the network’s MFD.

Xu, Guanhao↗
DOE OSTI2019

Electrostatic potential mapping at ferroelectric domain walls by low-temperature photoemission electron microscopy

Low-temperature X-ray photoemission electron microscopy (X-PEEM) is used to measure the electric potential at domain walls in improper ferroelectric Er 0.99 Ca 0.01 MnO 3 . Here, by combining X-PEEM with scanning probe microscopy and theory, we develop a model that relates the detected X-PEEM contrast to the emergence of uncompensated bound charges, explaining the image formation based on intrinsic electronic domain-wall properties. In contrast to previously applied low-temperature electrostatic force microscopy (EFM), X-PEEM readily distinguishes between positive and negative bound charges at domain walls. Finally, our study introduces an X-PEEM-based approach for low-temperature electrostatic potential mapping, facilitating nanoscale spatial resolution and data acquisition times on the order of 0.1-1 s.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Time resolved mechanism of the isotope selectivity in the ultrafast light induced dissociation in N 2

The time evolution of a vacuum ultraviolet excited N2 molecule is followed all the way from an ultrafast excitation to dissociation by a quantum mechanical simulation. The primary aim is to discern the role of the excitation by a pulse short compared to the vibrational period, to discern the different coupling mechanisms between different electronic states, nonadiabatic, spin orbit, and to analyze the origin of any isotopic effect. We compare the picture in the time and energy domains. The initial ultrafast excitation pumps the molecule to a coherent electronic wave packet to which several singlet bound electronic states contribute. The total nonstationary wave function is given as a coherent sum of nuclear wave packets on each electronic state times the stationary electronic wave function. When the wave packets on different electronic states overlap, they are coupled in a mass-dependent manner whether one uses an adiabatic or a diabatic electronic basis. A weak spin-orbit coupling acts as a bottleneck between the bound singlet part of phase space and the triplet manifold of states in which dissociation takes place. To describe the spin-orbit perturbation that is ongoing in time, an energy-resolved eigenstate representation appears to be more intuitive. In the eigenstate basis, the singlet-to-triplet population transfer is large only between those vibronic eigenstates that are quasiresonant in energy. The states in resonance are different for different excitation energy ranges. Finally, the resonances are mass dependent, which explains the control of the isotope effect through the profile of the pulse.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Photoelectron spectroscopy of [Mo6X14]2− dianions (X = Cl–I)

Photoelectron spectroscopy and theoretical investigations have been performed to systematically probe the intrinsic electronic properties of [Mo6X14]2− (X = halogen). All three PE spectra of gaseous [Mo6X14]2− (X = Cl, Br, I) dianions, which were generated by electrospray ionization, exhibit multiple resolved peaks in the recorded binding energy range. Theoretical investigations on the orbital structure and charge distribution were performed to support interpretation of the observed spectra and were further extended onto [Mo6F14]2−, a dianion that was not available for the experimental study. The measured adiabatic (ADE) and vertical detachment energies (VDE) for X = Cl–I were well reproduced by density functional theory calculations (accuracy ∼0.1 eV). Corresponding ADE/VDE values for the dianions were found to be 1.48/2.13 (calc.) and 2.30/2.65, 2.30/2.62, and 2.20/2.42 eV (all expt.) for X = F, Cl, Br, and I, respectively, showing an interesting buckled trend of electron binding energy (EBE) along the halogen series, i.e., EBE (F) ≪ EBE (Cl) ∼ EBE (Br) > EBE (I). Molecular orbital analyses indicate different mixing of metal and halogen atomic orbitals, which is strongly dependent on the nature of X, and suggest that the most loosely bound electrons are detached mainly from the metal core for X = F and Cl, but from halide ligands for X = Br and I. The repulsive Coulomb barrier (RCB), estimated from the photon energy dependent spectra, decreases with increasing halogen size, from 1.8 eV for X = Cl to 1.6 eV for X = I. Electrostatic potential modeling confirms the experimental RCB values and predicts that the most favorable electron detaching pathway should lie via the face-bridging halide ligands.

Li, Ren-Zhong↗
DOE OSTI2021

Effect of surface properties and polymer chain length on polymer adsorption in solution

In polymer nanoparticle composites (PNCs) with attractive interactions between nanoparticles (NPs) and polymers, a bound layer of the polymer forms on the NP surface, with significant effects on the macroscopic properties of the PNCs. The adsorption and wetting behaviors of polymer solutions in the presence of a solid surface are critical to the fabrication process of PNCs. In this study, we use both classical density functional theory (cDFT) and molecular dynamics (MD) simulations to study dilute and semi-dilute solutions of short polymer chains near a solid surface. Using cDFT, we calculate the equilibrium properties of polymer solutions near a flat surface while varying the solvent quality, surface–fluid interactions, and the polymer chain lengths to investigate their effects on the polymer adsorption and wetting transitions. Using MD simulations, we simulate polymer solutions near solid surfaces with three different curvatures (a flat surface and NPs with two radii) to study the static conformation of the polymer bound layer near the surface and the dynamic chain adsorption process. Additionally, we find that the bulk polymer concentration at which the wetting transition in the poor solvent system occurs is not affected by the difference in surface–fluid interactions; however, a threshold value of surface–fluid interaction is needed to observe the wetting transition. We also find that with good solvent, increasing the chain length or the difference in the surface–polymer interaction relative to the surface–solvent interaction increases the surface coverage of polymer segments and independent chains for all surface curvatures. Finally, we demonstrate that the polymer segmental adsorption times are heavily influenced only by the surface–fluid interactions, although polymers desorb more quickly from highly curved surfaces.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Photoluminescence study of solution-deposited Cu 2 BaSnS 4 thin films

To experimentally identify the character of radiative transitions in trigonal Cu 2 BaSnS 4 , we conduct temperature and excitation intensity dependent photoluminescence (PL) measurements in the temperature range of 15–300 K. The low-temperature near band edge PL spectrum is interpreted as the free exciton at 2.11 eV and the bound exciton at 2.08 eV, coupled with associated phonon-assisted transitions. In the low energy region, we assign the dominant defect emission at 1.96 eV to donor–acceptor-pair recombination and the weak broad emission at 1.6 eV to the free-to-bound transition. The activation energies and temperature shift for the radiative transitions are determined and discussed. Above 90 K, the free exciton recombination becomes the dominant radiative transition, with its energy shift mainly governed by the contribution of optical phonons.

36 MATERIALS SCIENCE↗
DOE OSTI2021

NENCI-2021. I. A large benchmark database of non-equilibrium non-covalent interactions emphasizing close intermolecular contacts

In this work, we present NENCI-2021, a benchmark database of ~8000 Non-Equilibirum Non-Covalent Interaction energies for a large and diverse selection of intermolecular complexes of biological and chemical relevance. To meet the growing demand for large and high-quality quantum mechanical data in the chemical sciences, NENCI-2021 starts with the 101 molecular dimers in the widely used S66 and S101 databases and extends the scope of these works by (i) including 40 cation–π and anion–π complexes, a fundamentally important class of non-covalent interactions that are found throughout nature and pose a substantial challenge to theory, and (ii) systematically sampling all 141 intermolecular potential energy surfaces (PESs) by simultaneously varying the intermolecular distance and intermolecular angle in each dimer. Designed with an emphasis on close contacts, the complexes in NENCI-2021 were generated by sampling seven intermolecular distances along each PES (ranging from 0.7× to 1.1× the equilibrium separation) and nine intermolecular angles per distance (five for each ion–π complex), yielding an extensive database of 7763 benchmark intermolecular interaction energies (E int ) obtained at the coupled-cluster with singles, doubles, and perturbative triples/complete basis set [CCSD(T)/CBS] level of theory. The E int values in NENCI-2021 span a total of 225.3 kcal/mol, ranging from -38.5 to +186.8 kcal/mol, with a mean (median) E int value of -1.06 kcal/mol (-2.39 kcal/mol). In addition, a wide range of intermolecular atom-pair distances are also present in NENCI-2021, where close intermolecular contacts involving atoms that are located within the so-called van der Waals envelope are prevalent—these interactions, in particular, pose an enormous challenge for molecular modeling and are observed in many important chemical and biological systems. A detailed symmetry-adapted perturbation theory (SAPT)- based energy decomposition analysis also confirms the diverse and comprehensive nature of the intermolecular binding motifs present in NENCI-2021, which now includes a significant number of primarily induction-bound dimers (e.g., cation–π complexes). NENCI-2021 thus spans all regions of the SAPT ternary diagram, thereby warranting a new four-category classification scheme that includes complexes primarily bound by electrostatics (3499), induction (700), dispersion (1372), or mixtures thereof (2192). A critical error analysis performed on a representative set of intermolecular complexes in NENCI-2021 demonstrates that the E int values provided herein have an average error of ±0.1 kcal/mol, even for complexes with strongly repulsive E int values, and maximum errors of ±0.2–0.3 kcal/mol (i.e., ~±1.0 kJ/mol) for the most challenging cases. For these reasons, we expect that NENCI-2021 will play an important role in the testing, training, and development of next-generation classical and polarizable force fields, density functional theory approximations, wavefunction theory methods, and machine learning based intra- and inter-molecular potentials.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗