Showing papers on "Ab initio published in 2016"

How van der Waals interactions determine the unique properties of water

Abstract: Whereas the interactions between water molecules are dominated by strongly directional hydrogen bonds (HBs), it was recently proposed that relatively weak, isotropic van der Waals (vdW) forces are essential for understanding the properties of liquid water and ice. This insight was derived from ab initio computer simulations, which provide an unbiased description of water at the atomic level and yield information on the underlying molecular forces. However, the high computational cost of such simulations prevents the systematic investigation of the influence of vdW forces on the thermodynamic anomalies of water. Here, we develop efficient ab initio-quality neural network potentials and use them to demonstrate that vdW interactions are crucial for the formation of water’s density maximum and its negative volume of melting. Both phenomena can be explained by the flexibility of the HB network, which is the result of a delicate balance of weak vdW forces, causing, e.g., a pronounced expansion of the second solvation shell upon cooling that induces the density maximum.

Potential energy surfaces from high fidelity fitting of ab initio points: the permutation invariant polynomial - neural network approach

Abstract: With advances in ab initio theory, it is now possible to calculate electronic energies within chemical (<1 kcal/mol) accuracy. However, it is still challenging to represent faithfully a large number of ab initio points with a multidimensional analytical function over a large configuration space, which is needed for accurate dynamical studies. In this Review, we discuss our recent work on a new potential-fitting approach based on artificial neural networks, which are ultra-flexible in representing any multidimensional real functions. A unique feature of our neural network approach is how the symmetries, particularly those associated with the exchange of identical atoms in the system, are enforced. To this end, symmetry functions in the form of symmetrised monomials that satisfy a particular type of symmetry possessed by the system are used in the input layer of the neural network. This approach is rigorous, accurate, and efficient. It is also simple to implement, requiring no modification of the neural net...

Interfacial Properties of Monolayer and Bilayer MoS2 Contacts with Metals: Beyond the Energy Band Calculations

Abstract: Although many prototype devices based on two-dimensional (2D) MoS2 have been fabricated and wafer scale growth of 2D MoS2 has been realized, the fundamental nature of 2D MoS2-metal contacts has not been well understood yet. We provide a comprehensive ab initio study of the interfacial properties of a series of monolayer (ML) and bilayer (BL) MoS2-metal contacts (metal = Sc, Ti, Ag, Pt, Ni, and Au). A comparison between the calculated and observed Schottky barrier heights (SBHs) suggests that many-electron effects are strongly suppressed in channel 2D MoS2 due to a charge transfer. The extensively adopted energy band calculation scheme fails to reproduce the observed SBHs in 2D MoS2-Sc interface. By contrast, an ab initio quantum transport device simulation better reproduces the observed SBH in 2D MoS2-Sc interface and highlights the importance of a higher level theoretical approach beyond the energy band calculation in the interface study. BL MoS2-metal contacts generally have a reduced SBH than ML MoS2-metal contacts due to the interlayer coupling and thus have a higher electron injection efficiency.

Graphene-like Two-Dimensional Ionic Boron with Double Dirac Cones at Ambient Condition

Abstract: Recently, partially ionic boron (γ-B28) has been predicted and observed in pure boron, in bulk phase and controlled by pressure [Nature 2009, 457, 863]. By using ab initio evolutionary structure search, we report the prediction of ionic boron at a reduced dimension and ambient pressure, namely, the two-dimensional (2D) ionic boron. This 2D boron structure consists of graphene-like plane and B2 atom pairs with the P6/mmm space group and six atoms in the unit cell and has lower energy than the previously reported α-sheet structure and its analogues. Its dynamical and thermal stability are confirmed by the phonon-spectrum and ab initio molecular dynamics simulation. In addition, this phase exhibits double Dirac cones with massless Dirac Fermions due to the significant charge transfer between the graphene-like plane and B2 pair that enhances the energetic stability of the P6/mmm boron. A Fermi velocity (vf) as high as 2.3 × 106 m/s, which is even higher than that of graphene (0.82 × 106 m/s), is predicted for...

Potential Dependence of Electrochemical Barriers from ab Initio Calculations

Abstract: We present a simple and computationally efficient method to determine the potential dependence of the activation energies for proton–electron transfer from a single ab initio barrier calculation. We show that the potential dependence of the activation energy is given by the partial charge transferred at the transition state. The method is evaluated against the potential dependence determined explicitly through multiple calculations at varying potential. We show that the transfer coefficient is given by the charge transferred from the initial to transition state, which has significant implications for electrochemical kinetics.

A computational chemist's guide to accurate thermochemistry for organic molecules

Abstract: Composite ab initio methods are multistep theoretical procedures specifically designed to obtain highly accurate thermochemical and kinetic data with confident sub-kcal mol−1 or sub-kJ mol−1 accuracy. These procedures include all energetic terms that contribute to the molecular binding energies at these levels of accuracy (e.g., CCSD(T), post-CCSD(T), core–valence, relativistic, spin-orbit, Born–Oppenheimer, and zero-point vibrational energy corrections). Basis-set extrapolations (and other basis-set acceleration techniques) are used for obtaining these terms at sufficiently high levels of accuracy. Major advances in computer hardware and theoretical methodologies over the past two decades have enabled the application of these procedures to medium-sized organic systems (e.g., ranging from benzene and hexane to amino acids and DNA bases). With these advances, there has been a proliferation in the number of developed composite ab initio methods. We give an overview of the accuracy and applicability of the various types of composite ab initio methods that were developed in recent years. General recommendations to guide selection of the most suitable method for a given problem are presented, with a special emphasis on organic molecules. WIREs Comput Mol Sci 2016, 6:292–310. doi: 10.1002/wcms.1249 For further resources related to this article, please visit the WIREs website.

Discovering chemistry with an ab initio nanoreactor

Abstract: Chemical understanding is driven by the experimental discovery of new compounds and reactivity, and is supported by theory and computation that provides detailed physical insight. While theoretical and computational studies have generally focused on specific processes or mechanistic hypotheses, recent methodological and computational advances harken the advent of their principal role in discovery. Here we report the development and application of the ab initio nanoreactor – a highly accelerated, first-principles molecular dynamics simulation of chemical reactions that discovers new molecules and mechanisms without preordained reaction coordinates or elementary steps. Using the nanoreactor we show new pathways for glycine synthesis from primitive compounds proposed to exist on the early Earth, providing new insight into the classic Urey-Miller experiment. These results highlight the emergence of theoretical and computational chemistry as a tool for discovery in addition to its traditional role of interpreting experimental findings.

Ab Initio Study of Phosphorus Anodes for Lithium- and Sodium-Ion Batteries

Abstract: Phosphorus has received recent attention in the context of high-capacity and high-rate anodes for lithium- and sodium-ion batteries. Here, we present a first-principles structure prediction study combined with NMR calculations, which gives us insights into its lithiation/sodiation process. We report a variety of new phases found by the ab initio random structure searching (AIRSS) and the atomic species swapping methods. Of particular interest, a stable Na5P4–C2/m structure and locally stable structures found less than 10 meV/f.u. from the convex hull such as Li4P3–P212121, NaP5–Pnma, and Na4P3–Cmcm. The mechanical stability of Na5P4–C2/m and Li4P3–P212121 has been studied by first-principles phonon calculations. We have calculated average voltages, which suggest that black phosphorus (BP) can be considered as a safe anode in lithium-ion batteries due to its high lithium insertion voltage, 1.5 V; moreover, BP exhibits a relatively low theoretical volume expansion compared with other intercalation anodes, 2...

Ab initio electron mobility and polar phonon scattering in GaAs

Abstract: In polar semiconductors and oxides, the long-range nature of the electron-phonon (e−ph) interaction is a bottleneck to compute charge transport from first principles. Here, we develop an efficient ab initio scheme to compute and converge the e−ph relaxation times (RTs) and electron mobility in polar materials. We apply our approach to GaAs, where by using the Boltzmann equation with state-dependent RTs, we compute mobilities in excellent agreement with experiment at 250–500K. The e−ph RTs and the phonon contributions to intravalley and intervalley e−ph scattering are also analyzed. Our work enables efficient ab initio computations of transport and carrier dynamics in polar materials.

Two-dimensional Fröhlich interaction in transition-metal dichalcogenide monolayers: Theoretical modeling and first-principles calculations

Abstract: We perform ab initio calculations of the coupling between electrons and small-momentum polar-optical phonons in monolayer transition-metal dichalcogenides of the $2H$ type: ${\mathrm{MoS}}_{2},\phantom{\rule{0.28em}{0ex}}{\mathrm{MoSe}}_{2},\phantom{\rule{0.28em}{0ex}}{\mathrm{MoTe}}_{2},\phantom{\rule{0.28em}{0ex}}{\mathrm{WS}}_{2}$, and ${\mathrm{WSe}}_{2}$. The polar-optical coupling with longitudinal optical phonons, or Fr\"ohlich interaction, is fundamentally affected by the dimensionality of the system. In a plane-wave framework with periodic boundary conditions, the Fr\"ohlich interaction is affected by the spurious interaction between the two-dimensional (2D) material and its periodic images. To overcome this difficulty, we perform density functional perturbation theory calculations with a truncated Coulomb interaction in the direction perpendicular to the plane of the 2D material. We show that the two-dimensional Fr\"ohlich interaction is much stronger than assumed in previous ab initio studies. We provide analytical models depending on the effective charges and dielectric properties of the materials to interpret our ab initio calculations. Screening is shown to play a fundamental role in the phonon-momentum dependency of the polar-optical coupling, with a crossover between two regimes depending on the dielectric properties of the material relative to its environment. The Fr\"ohlich interaction is screened by the dielectric environment in the limit of small phonon momenta and sharply decreases due to stronger screening by the monolayer at finite momenta. The small-momentum regime of the ab initio Fr\"ohlich interaction is reproduced by a simple analytical model, for which we provide the necessary parameters. At larger momenta, however, direct ab initio calculations of electron-phonon interactions are necessary to capture band-specific effects. We compute and compare the carrier relaxation times associated with the scattering by both LO and ${A}_{1}$ phonon modes. While both modes are capable of relaxing carriers on time scales under the picosecond at room temperature, their absolute importance and relative importance vary strongly depending on the material, the band, and the substrate.

The S66x8 benchmark for noncovalent interactions revisited: explicitly correlated ab initio methods and density functional theory

Abstract: The S66x8 dataset for noncovalent interactions of biochemical relevance has been re-examined by means of MP2-F12 and CCSD(F12*)(T) methods. We deem our revised benchmark data to be reliable to about 0.05 kcal mol−1 RMS. Most levels of DFT perform quite poorly in the absence of dispersion corrections: somewhat surprisingly, that is even the case for the double hybrids and for dRPA75. Analysis of optimized D3BJ parameters reveals that the main benefit of dRPA75 and DSD double hybrids alike is the treatment of midrange dispersion. dRPA75-D3BJ is the best performer overall at RMSD = 0.10 kcal mol−1. The nonlocal VV10 dispersion functional is especially beneficial for the double hybrids, particularly in DSD-PBEP86-NL (RMSD = 0.12 kcal mol−1). Other recommended dispersion-corrected functionals with favorable price/performance ratios are ωB97X-V, and, surprisingly, B3LYP-D3BJ and BLYP-D3BJ (RMSDs of 0.23, 0.20 and 0.23 kcal mol−1, respectively). Without dispersion correction (but parametrized for midrange interactions) M06-2X has the lead (RMSD = 0.45 kcal mol−1). A collection of three energy-based diagnostics yields similar information to an SAPT analysis about the nature of the noncovalent interaction. Two of those are the percentages of Hartree–Fock and of post-MP2 correlation effects in the interaction energy; the third, CSPI = [IE(2)ss − IE(2)ab]/[IE(2)ss + IE(2)ab] or its derived quantity DEBC = CSPI/(1 + CSPI2)1/2, describes the character of the MP2 correlation contribution, ranging from 0 (purely dispersion) to 1 (purely other effects). In addition, we propose an improved, parameter-free scaling for the (T) contribution based on the Ecorr[CCSD-F12b]/Ecorr[CCSD] and Ecorr[CCSD(F12*)]/Ecorr[CCSD] ratios. For Hartree–Fock and conventional DFT calculations, full counterpoise generally yields the fastest basis set convergence, while for double hybrids, half-counterpoise yields faster convergence, as previously established for correlated ab initio methods.

Electron-Phonon Coupling and Energy Flow in a Simple Metal beyond the Two-Temperature Approximation

Abstract: The electron-phonon coupling and the corresponding energy exchange are investigated experimentally and by ab initio theory in nonequilibrium states of the free-electron metal aluminium. The temporal evolution of the atomic mean-squared displacement in laser-excited thin freestanding films is monitored by femtosecond electron diffraction. The electron-phonon coupling strength is obtained for a range of electronic and lattice temperatures from density functional theory molecular dynamics simulations. The electron-phonon coupling parameter extracted from the experimental data in the framework of a twotemperature model (TTM) deviates significantly from the ab initio values. We introduce a nonthermal lattice model (NLM) for describing nonthermal phonon distributions as a sum of thermal distributions of the three phonon branches. The contributions of individual phonon branches to the electron-phonon coupling are considered independently and found to be dominated by longitudinal acoustic phonons. Using all material parameters from first-principles calculations except the phonon-phonon coupling strength, the prediction of the energy transfer from electrons to phonons by the NLM is in excellent agreement with time-resolved diffraction data. Our results suggest that the TTM is insufficient for describing the microscopic energy flow even for simple metals like aluminium and that the determination of the electronphonon coupling constant from time-resolved experiments by means of the TTM leads to incorrect values. In contrast, the NLM describing transient phonon populations by three parameters appears to be a sufficient model for quantitatively describing electron-lattice equilibration in aluminium. We discuss the general applicability of the NLM and provide a criterion for the suitability of the two-temperature approximation for other metals.

Ab initio phonon coupling and optical response of hot electrons in plasmonic metals

Abstract: Ultrafast laser measurements probe the nonequilibrium dynamics of excited electrons in metals with increasing temporal resolution. Electronic structure calculations can provide a detailed microscopic understanding of hot electron dynamics, but a parameter-free description of pump-probe measurements has not yet been possible, despite intensive research, because of the phenomenological treatment of electron-phonon interactions. We present ab initio predictions of the electron-temperature dependent heat capacities and electron-phonon coupling coefficients of plasmonic metals. We find substantial differences from free-electron and semiempirical estimates, especially in noble metals above transient electron temperatures of 2000 K, because of the previously neglected strong dependence of electron-phonon matrix elements on electron energy. We also present first-principles calculations of the electron-temperature dependent dielectric response of hot electrons in plasmonic metals, including direct interband and phonon-assisted intraband transitions, facilitating complete theoretical predictions of the time-resolved optical probe signatures in ultrafast laser experiments.

Matrix product operators, matrix product states, and ab initio density matrix renormalization group algorithms

Abstract: Current descriptions of the ab initio density matrix renormalization group (DMRG) algorithm use two superficially different languages: an older language of the renormalization group and renormalized operators, and a more recent language of matrix product states and matrix product operators. The same algorithm can appear dramatically different when written in the two different vocabularies. In this work, we carefully describe the translation between the two languages in several contexts. First, we describe how to efficiently implement the ab initio DMRG sweep using a matrix product operator based code, and the equivalence to the original renormalized operator implementation. Next we describe how to implement the general matrix product operator/matrix product state algebra within a pure renormalized operator-based DMRG code. Finally, we discuss two improvements of the ab initio DMRG sweep algorithm motivated by matrix product operator language: Hamiltonian compression, and a sum over operators representation that allows for perfect computational parallelism. The connections and correspondences described here serve to link the future developments with the past and are important in the efficient implementation of continuing advances in ab initio DMRG and related algorithms.

Unified ab initio approaches to nuclear structure and reactions

Abstract: The description of nuclei starting from the constituent nucleons and the realistic interactions among them has been a long-standing goal in nuclear physics. In addition to the complex nature of the nuclear forces, with two-, three- and possibly higher many-nucleon components, one faces the quantum-mechanical many-nucleon problem governed by an interplay between bound and continuum states. In recent years, significant progress has been made in ab initio nuclear structure and reaction calculations based on input from QCD-employing Hamiltonians constructed within chiral effective field theory. After a brief overview of the field, we focus on ab initio many-body approaches - built upon the No-Core Shell Model - that are capable of simultaneously describing both bound and scattering nuclear states, and present results for resonances in light nuclei, reactions important for astrophysics and fusion research. In particular, we review recent calculations of resonances in the $^6$He halo nucleus, of five- and six-nucleon scattering, and an investigation of the role of chiral three-nucleon interactions in the structure of $^9$Be. Further, we discuss applications to the $^7$Be$(p,\gamma)^8$B radiative capture. Finally, we highlight our efforts to describe transfer reactions including the $^3$H$(d,n)^4$He fusion.

Defect energetics of concentrated solid-solution alloys from ab initio calculations: Ni0.5Co0.5, Ni0.5Fe0.5, Ni0.8Fe0.2 and Ni0.8Cr0.2

Abstract: It has been shown that concentrated solid solution alloys possess unusual electronic, magnetic, transport, mechanical and radiation-resistant properties that are directly related to underlying chemical complexity. Because every atom experiences a different local atomic environment, the formation and migration energies of vacancies and interstitials in these alloys exhibit a distribution, rather than a single value as in a pure metal or dilute alloy. Using ab initio calculations based on density functional theory and special quasirandom structures, we have characterized the distribution of defect formation energy and migration barrier in four Ni-based solid-solution alloys: Ni0.5Co0.5, Ni0.5Fe0.5, Ni0.8Fe0.2, and Ni0.8Cr0.2. As defect formation energies in finite-size models depend sensitively on the elemental chemical potential, we have developed a computationally efficient method for determining it which takes into account the global composition and the local short-range order. In addition we have compared the results of our ab initio calculations to those obtained from available embedded atom method (EAM) potentials. Our results indicate that the defect formation and migration energies are closely related to the specific atoms in the structure, which further determines the elemental diffusion properties. Different EAM potentials yield different features of defect energetics in concentrated alloys, pointing to the need for additional potential development efforts in order to allow spatial and temporal scale-up of defect and simulations, beyond those accessible to ab initio methods.

Structural Transformation of MXene (V2C, Cr2C, and Ta2C) with O Groups during Lithiation: A First-Principles Investigation.

Abstract: For high capacities and extremely fast charging rates, two-dimensional (2D) crystals exhibit a significant promising application on lithium-ion batteries. With density functional calculations, this paper systematically investigated the Li storage properties of eight 2D M2CO2 (M = V, Cr, Ta, Sc, Ti, Zr, Nb, and Hf), which are the recently synthesized transition-metal carbides (called MXenes) with O groups. According to whether the structural transformation occurs or not during the adsorption of the first Li layer, the adsorption of Li can be grouped into two types: V-type (V2CO2, Cr2CO2, and Ta2CO2) and Sc-type (Sc2CO2, Ti2CO2, Zr2CO2, Nb2CO2, and Hf2CO2). The structural transformation behaviors of V-type are reversible during lithiation/delithiation and are confirmed by ab initio molecular dynamic simulations. Except for Nb-MXene, the V-type prefers the sandwich H2H1T-M2CO2Li4 structure and the Sc-type prefers the TH1H2-M2CO2Li4 structure during the adsorption of the second Li layer. The H2H1T-M2CO2Li4 st...

Metallic VS2 Monolayer Polytypes as Potential Sodium-Ion Battery Anode via ab Initio Random Structure Searching

Abstract: We systematically investigated the potential of single-layer VS2 polytypes as Na-battery anode materials via density functional theory calculations. We found that sodiation tends to inhibit the 1H-to-1T structural phase transition, in contrast to lithiation-induced transition on monolayer MoS2. Thus, VS2 can have better structural stability in the cycles of charging and discharging. Diffussion of Na atom was found to be very fast on both polytypes, with very small diffusion barriers of 0.085 eV (1H) and 0.088 eV (1T). Ab initio random structure searching was performed in order to explore stable configurations of Na on VS2. Our search found that both the V top and the hexagonal center sites are preferred adsorption sites for Na, with the 1H phase showing a relatively stronger binding. Notably, our random structures search revealed that Na clusters can form as a stacked second layer at full Na concentration, which is not reported in earlier works wherein uniform, single-layer Na adsorption phases were assum...

Moderate Humidity Delays Electron-Hole Recombination in Hybrid Organic-Inorganic Perovskites: Time-Domain Ab Initio Simulations Rationalize Experiments.

Abstract: Experiments show both positive and negative changes in performance of hybrid organic–inorganic perovskite solar cells upon exposure to moisture. Ab initio nonadiabatic molecular dynamics reveals the influence of humidity on nonradiative electron–hole recombination. In small amounts, water molecules perturb perovskite surface and localize photoexcited electron close to the surface. Importantly, deep electron traps are avoided. The electron–hole overlap decreases, and the excited state lifetime increases. In large amounts, water forms stable hydrogen-bonded networks, has a higher barrier to enter perovskite, and produces little impact on charge localization. At the same time, by contributing high frequency polar vibrations, water molecules increase nonadiabatic coupling and accelerate recombination. In general, short coherence between electron and hole benefits photovoltaic response of the perovskites. The calculated recombination time scales show excellent agreement with experiment. The time-domain atomist...

Ab initio structure determination of nanocrystals of organic pharmaceutical compounds by electron diffraction at room temperature using a Timepix quantum area direct electron detector

Abstract: Until recently, structure determination by transmission electron microscopy of beam-sensitive three-dimensional nanocrystals required electron diffraction tomography data collection at liquid-nitrogen temperature, in order to reduce radiation damage. Here it is shown that the novel Timepix detector combines a high dynamic range with a very high signal-to-noise ratio and single-electron sensitivity, enabling ab initio phasing of beam-sensitive organic compounds. Low-dose electron diffraction data (∼0.013 e− A−2 s−1) were collected at room temperature with the rotation method. It was ascertained that the data were of sufficient quality for structure solution using direct methods using software developed for X-ray crystallography (XDS, SHELX) and for electron crystallography (ADT3D/PETS, SIR2014).

Nonadiabatic Tunneling in Photodissociation of Phenol

Abstract: Using recently developed full-dimensional coupled quasi-diabatic ab initio potential energy surfaces including four electronic (1ππ, 1ππ*, 11πσ*, and 21πσ*) states, the tunneling dynamics of phenol photodissociation via its first excited singlet state (S1 ← S0) is investigated quantum mechanically using a three-dimensional model. The lifetimes of several low-lying vibrational states are examined and compared with experiment. The deuteration of the phenoxyl hydrogen is found to dramatically increase the lifetime, attesting to the tunneling nature of the nonadiabatic dissociation. Importantly, it is shown that owing to the conical intersection topography tunneling in this system cannot be described in the standard adiabatic approximation, which eschews the geometric phase effect since the nonadiabatically computed lifetimes, validated by comparison with experiment, differ significantly from those obtained in that limit.

Ab initio calculations of grain boundaries in bcc metals

Abstract: In this study, we compute grain boundary (GB) properties for a large set of GBs in bcc transition metals with a special focus on W, Mo and Fe using ab initio density functional theory (DFT) and semi-empirical second nearest neighbour modified embedded atom method (2NN-MEAM) potentials. The GB properties include GB energies, surface energies, GB excess volume and work of separation, which we analyse and then compare to experimental data. We find that the used 2NN-MEAM potentials can predict general trends of GB properties, but do not always reproduce the GB ground state structure and energy found with DFT. In particular, our results explain the experimental finding that W and Mo prefer intergranular fracture, while other bcc metals prefer transgranular cleavage.

Amorphous SiO2 surface models: energetics of the dehydroxylation process, strain, ab initio atomistic thermodynamics and IR spectroscopic signatures

Abstract: In this contribution, realistic amorphous SiO2 models of 2.1 × 2.1 nm with silanol densities ranging 1.1–7.2 OH per nm2 are obtained by means of ab initio calculations via the dehydroxylation of a fully hydroxylated silica surface. The dehydroxyation process is considered to take place via direct condensation of adjacent silanol groups and silica migration steps. The latter reconstructions are needed in order to obtain highly dehydroxylated silica surfaces with favorable energetics and without the formation of defects. The obtained surface phase diagram of different silica models as a function of temperature and PH2O is able to correctly describe the silanol density under different conditions, and the IR spectroscopic signatures of the silanols are in qualitative agreement with the experiment. The amorphous silica models presented here have a high degree of heterogeneity as found from the big variability obtained in the energetics of the dehydroxylation steps. It was also found that the resulting average Si–O distance of the newly formed siloxane bridges serves as a descriptor of the strain introduced in the silica surface. All these factors can be crucial in order to simulate the activity of catalysts grafted onto silica with different silanol densities, especially the one containing ca. 1 OH per nm2, which can serve as a model for the SiO2 surface pretreated under high vacuum and at 700 °C.

Ground and excited states of doubly open-shell nuclei from ab initio valence-space Hamiltonians

Abstract: We present ab initio predictions for ground and excited states of doubly open-shell fluorine and neon isotopes based on chiral two- and three-nucleon interactions. We use the in-medium similarity renormalization group to derive mass-dependent $sd$ valence-space Hamiltonians. The experimental ground-state energies are reproduced through neutron number $N=14$, beyond which a new targeted normal-ordering procedure improves agreement with data and large-space multireference calculations. For spectroscopy, we focus on neutron-rich $^{23\ensuremath{-}26}\mathrm{F}$ and $^{24\ensuremath{-}26}\mathrm{Ne}$ isotopes near $N=14,16$ magic numbers. In all cases we find agreement with experiment and established phenomenology. Moreover, yrast states are well described in $^{20}\mathrm{Ne}$ and $^{24}\mathrm{Mg}$, providing a path toward an ab initio description of deformation in the medium-mass region.

A room temperature CO2 line list with ab initio computed intensities

Abstract: Atmospheric carbon dioxide concentrations are being closely monitored by remote sensing experiments which rely on knowing line intensities with an uncertainty of 0.5% or better. We report a theoretical study providing rotation–vibration line intensities substantially within the required accuracy based on the use of a highly accurate ab initio dipole moment surface (DMS). The theoretical model developed is used to compute CO2 intensities with uncertainty estimates informed by cross comparing line lists calculated using pairs of potential energy surfaces (PES) and DMS׳s of similar high quality. This yields lines sensitivities which are utilized in reliability analysis of our results. The final outcome is compared to recent accurate measurements as well as the HITRAN2012 database. Transition frequencies are obtained from effective Hamiltonian calculations to produce a comprehensive line list covering all 12C16O2 transitions below 8000 cm − 1 and stronger than 10 − 30 cm /molecule at T = 296 K .

Ab Initio Thermodynamics and First-Principles Microkinetics for Surface Catalysis

Abstract: Ab initio thermodynamics and first-principles microkinetic simulations have become standard tools in research on model catalysts. Complementing dedicated in situ experiments these techniques contribute to our evolving mechanistic understanding, in particular of a reaction-induced dynamical evolution of the working catalyst surface. This topical review surveys the methodological foundations and ongoing developments of both techniques, and specifically illustrates the type of insights they provide in the context of in situ model catalyst studies. This insight points at substantial deviations from the standard picture that analyzes catalytic function merely in terms of properties of and processes at active sites as they emerge from a crystal lattice truncation of the nominal catalyst bulk material.

Solution-based intramolecular singlet fission in cross-conjugated pentacene dimers.

Abstract: We show unambiguous and compelling evidence by means of pump-probe experiments, which are complemented by calculations using ab initio multireference perturbation theory, for intramolecular singlet fission (SF) within two synthetically tailored pentacene dimers with cross-conjugation, namely XC1 and XC2. The two pentacene dimers differ in terms of electronic interactions as evidenced by perturbation of the ground state absorption spectra stemming from stronger through-bond contributions in XC1 as confirmed by theory. Multiwavelength analysis, on one hand, and global analysis, on the other hand, confirm that the rapid singlet excited state decay and triplet excited state growth relate to SF. SF rate constants and quantum yields increase with solvent polarity. For example, XC2 reveals triplet quantum yields and rate constants as high as 162 ± 10% and (0.7 ± 0.1) × 10(12) s(-1), respectively, in room temperature solutions.

Benchmark ab Initio Conformational Energies for the Proteinogenic Amino Acids through Explicitly Correlated Methods. Assessment of Density Functional Methods

Abstract: The relative energies of the YMPJ conformer database of the 20 proteinogenic amino acids, with N- and C-termination, have been re-evaluated using explicitly correlated coupled cluster methods. Lower-cost ab initio methods such as MP2-F12 and CCSD-F12b actually are outperformed by double-hybrid DFT functionals; in particular, the DSD-PBEP86-NL double hybrid performs well enough to serve as a secondary standard. Among range-separated hybrids, ωB97X-V performs well, while B3LYP-D3BJ does surprisingly well among traditional DFT functionals. Treatment of dispersion is important for the DFT functionals; for the YMPJ set, D3BJ generally works as well as the NL nonlocal dispersion functional. Basis set sensitivity for DFT calculations on these conformers is weak enough that def2-TZVP is generally adequate. For conformer corrections to heats of formation, B3LYP-D3BJ and especially DSD-PBEP86-D3BJ or DSD-PBEP86-NL are adequate for all but the most exacting applications. The revised geometries and energetics for the...

Microscopic theory and quantum simulation of atomic heat transport

Abstract: Quantum simulation methods based on electronic-structure theory are deemed unfit to cope with atomic heat transport within the Green–Kubo formalism, because quantum-mechanical energy densities and currents are inherently ill-defined at the atomic scale. We show that, although this difficulty would also affect classical simulations, thermal conductivity is indeed insensitive to such ill-definedness by virtue of a kind of gauge invariance resulting from energy extensivity and conservation. On the basis of these findings, we derive an expression for the adiabatic energy flux from density-functional theory, which allows heat transport to be simulated using ab initio equilibrium molecular dynamics. Our methodology is demonstrated by comparing its predictions to those of classical equilibrium and ab initio non-equilibrium (Muller–Plathe) simulations for a liquid-argon model, and by applying it to heavy water at ambient conditions. Heat transport is well described by the Green–Kubo formalism. Now, the formalism is combined with density-functional theory, enabling simulations of thermal conduction in systems that cannot be adequately modelled by classical interatomic potentials.