N2 Dimers Research Articles

FragPT2: Multifragment Wave Function Embedding with Perturbative Interactions.

Embedding techniques allow the efficient description of correlations within localized fragments of large molecular systems while accounting for their environment at a lower level of theory. We introduce FragPT2: a novel embedding framework that addresses multiple interacting active fragments. Fragments are assigned separate active spaces, constructed by localizing canonical molecular orbitals. Each fragment is then solved with a multireference method, self-consistently embedded in the mean field from other fragments. Finally, interfragment correlations are reintroduced through multireference perturbation theory. Our framework provides an exhaustive classification of interfragment interaction terms, offering a tool to analyze the relative importance of various processes such as dispersion, charge transfer, and spin exchange. We benchmark FragPT2 on challenging test systems, including N2 dimers, multiple aromatic dimers, and butadiene. We demonstrate that our method can be successful even for fragments defined by cutting through a covalent bond.

Quantum Algorithms for the Variational Optimization of Correlated Electronic States with Stochastic Reconfiguration and the Linear Method.

Solving the electronic Schrodinger equation for strongly correlated ground states is a long-standing challenge. We present quantum algorithms for the variational optimization of wave functions correlated by products of unitary operators, such as Local Unitary Cluster Jastrow (LUCJ) ansatzes, using stochastic reconfiguration (SR) and the linear method (LM). While an implementation on classical computing hardware would require exponentially growing compute cost, the cost (number of circuits and shots) of our quantum algorithms is polynomial in system size. We find that classical simulations of optimization with the linear method consistently find lower energy solutions than with the L-BFGS-B optimizer across the dissociation curves of the notoriously difficult N2 and C2 dimers; LUCJ predictions of the ground-state energies deviate from exact diagonalization by 1 kcal/mol or less at all points on the potential energy curve. While we do characterize the effect of shot noise on the LM optimization, these noiseless results highlight the critical but often overlooked role that optimization techniques must play in attacking the electronic structure problem (on both classical and quantum hardware), for which even mean-field optimization is formally NP hard. We also discuss the challenge of obtaining smooth curves in these strongly correlated regimes, and propose a number of quantum-friendly solutions ranging from symmetry-projected ansatz forms to a symmetry-constrained optimization algorithm.

Toward Large-Scale AFQMC Calculations: Large Time Step Auxiliary-Field Quantum Monte Carlo.

We report modifications of the ph-AFQMC algorithm that allow the use of large time steps and reliable time step extrapolation. Our modified algorithm eliminates size-consistency errors present in the standard algorithm when large time steps are employed. We investigate various methods to approximate the exponential of the one-body operator within the AFQMC framework, distinctly demonstrating the superiority of Krylov methods over the conventional Taylor expansion. We assess various propagators within AFQMC and demonstrate that the Split-2 propagator is the optimal method, exhibiting the smallest time-step errors. For the HEAT set molecules, the time-step extrapolated energies deviate on average by only 0.19 kcal/mol from the accurate small time-step energies. For small water clusters, we obtain accurate complete basis-set binding energies using time-step extrapolation with a mean absolute error of 0.07 kcal/mol compared to CCSD(T). Using large time-step ph-AFQMC for the N2 dimer, we show that accurate bond lengths can be obtained while reducing CPU time by an order of magnitude.

Pressure-Driven Ne-Bearing Polynitrides with Ultrahigh Energy Density

Neon (Ne) can reveal the evolution of planets, and nitrogen (N) is the most abundant element in the Earth’s atmosphere. Considering the inertness of neon, whether nitrogen and neon can react has aroused great interest in condensed matter physics and space science. Here, we identify three new Ne–N compounds (i.e., NeN6, NeN10, and NeN22) under pressure by first-principles calculations. We find that inserting Ne into N2 substantially decreases the polymeric pressure of the nitrogen and promotes the formation of abundant polynitrogen structures. Especially, NeN22 acquires a duplex host-guest structure, in which guest atoms (Ne and N2 dimers) are trapped inside the crystalline host N20 cages. Importantly, both NeN10 and NeN22 not only are dynamically and mechanically stable but also have a high thermal stability up to 500 K under ambient pressure. Moreover, ultra-high energy densities are obtained in NeN10 (11.1 kJ/g), NeN22 (11.5 kJ/g), tetragonal t-N22 (11.6 kJ/g), and t-N20 (12.0 kJ/g) produced from NeN22, which are more than twice the value of trinitrotoluene (TNT). Meanwhile, their explosive performance is superior to that of TNT. Therefore, NeN10, NeN22, t-N22, and t-N20 are promising green high-energy-density materials. This work promotes the study of neon-nitrogen compounds with superior properties and potential applications.

Erratum: Large spin gaps in the half-metals MN4(M=Mn,Fe,Co) with N2 dimers [Phys. Rev. B 99 , 184409 (2019)

Erratum: Large spin gaps in the half-metals <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mrow><mml:mi>M</mml:mi><mml:mi mathvariant="normal">N</mml:mi></mml:mrow><mml:mn>4</mml:mn></mml:msub><mml:mspace width="0.28em" /><mml:mrow><mml:mo>(</mml:mo><mml:mi>M</mml:mi><mml:mo>=</mml:mo><mml:mrow><mml:mi>Mn</mml:mi><mml:mo>,</mml:mo><mml:mi>Fe</mml:mi><mml:mo>,</mml:mo><mml:mi>Co</mml:mi></mml:mrow><mml:mo>)</mml:mo></mml:mrow></mml:mrow></mml:math> with <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi mathvariant="normal">N</mml:mi><mml:mn>2</mml:mn></mml:msub></mml:math> dimers [Phys. Rev. B <b>99</b> , 184409 (2019)

Correction: Quantum tunneling dynamical behaviour on weakly bound complexes: the case of a CO2–N2 dimer

Correction for 'Quantum tunneling dynamical behaviour on weakly bound complexes: the case of a CO2-N2 dimer' by Miguel Lara-Moreno et al., Phys. Chem. Chem. Phys., 2019, 21, 3550-3557, DOI: 10.1039/c8cp04465a.

Ultra-incompressibility and high energy density of ReN8 with infinite nitrogen chains

Nitrogen-rich transition metal nitrides are of great interests due to the unique physical and chemical properties. Here, we perform a detailed structural investigation of ReN8 in the pressure range of 0–200 GPa via particle swarm optimization algorithm and first-principles calculations. Interestingly, four ReN8 phases are firstly revealed to be stable within 200 GPa, N2 dimers are found to be preferred in T-ReN8 at pressure lower than 15.8 GPa and the infinite N∞ chains become dominated in M-, T’- and M’-ReN8 at higher pressure up to 200 GPa. We find that the Young’s moduli of M- and T’-ReN8, about 850 GPa along [0.88 0 0.47] and [0 0 1] direction, are comparable to cubic BN, due to the presence of electronic accumulation in the N∞ chains. Furthermore, M- and T’-ReN8 are found to be of high energy density of ~ 1.9 kJ/g (~ 11.0 kJ/cm3). Ultra-incompressibility of ReN8 in special direction due to the high electronic accumulations in the polynitrogen chains.

High Curie temperature ferromagnetism in penta-MnN2 monolayer

Two-dimensional (2D) intrinsic ferromagnetic materials with high Curie temperature have attracted increasing research interest because of their potential applications in spintronics. Here, on the basis of first principles calculations combined with Monte Carlo simulation, we report a new 2D pentagon-based monolayer, penta-MnN2, which is not only dynamically, mechanically and thermally stable, but also exhibits half-metallicity with a band-gap of 1.12 eV in spin-down channel. More interestingly, penta-MnN2 displays a high Curie temperature TC of 913 K that can be further improved to 956 K when the biaxial tensile strain of 3% is applied. Compared to the reported hexagonal MnN monolayer, the exchange interaction between the Mn atoms is significantly enhanced in the penta-MnN2 sheet due to its unique geometric configuration composed solely of the pentagons containing N2 dimers.

First-principles calculation of correlated electron materials based on Gutzwiller wave function beyond Gutzwiller approximation

We propose an approach that is under the framework of Gutzwiller wave function but goes beyond the commonly adopted Gutzwiller approximation to improve the accuracy and flexibility in treating the correlation effects. Detailed formalism is described for a dimer which is straightforwardly generalized later to more complicated periodic bulk systems. The accuracy of the approach is demonstrated by evaluating the potential energy curves of spin-singlet N2 dimer, spin-triplet O2 dimer, and 1D hydrogen chain. The computational workload of the approach can be easily handled by efficient parallel computing.

Large spin gaps in the half-metals MN4 ( M =Mn, Fe, Co) with N2 dimers

We predict that cubic MN4 (M=Mn, Fe, Co) are all half metals with the largest spin gap up to ~ 5 eV. They possess robust ferromagnetic ground states with the highest Curie temperature up to ~ 103 K. Our calculations indicate these compounds are energetically favored, dynamically and mechanically stable. It is proposed that self-doping of these 3d transition metals occurs in MN4 due to the reduction in electronegativity of N2 dimers. This model can well explain the calculated integer magnetic moments, large spin gaps of MN4 and semiconducting behavior for NiN4 as well. Our results highlight the difference in electronegativity between transition metal ions and non-metal entities in forming half metals and the role of N2 dimer in enlarging the spin gaps for nitride half metals.

2D planar penta-MN2 (M = Pd, Pt) sheets identified through structure search.

Two-dimensional (2D) metal dinitrides have attracted increasing attention because of their diverse geometry configurations and extraordinary properties. Using the particle swarm optimization (PSO)-based global structure search method combined with first principles calculations, we have identified 2D metal dinitride MN2 (M = Pd, Pt) sheets containing N2 dimers, penta-MN2, which are planar and composed entirely of pentagons. These pentagonal sheets are not only dynamically, thermally and mechanically stable, but also energetically more favorable over the experimentally synthesized pyrite MN2 monolayers. In particular, penta-PtN2 can withstand a temperature as high as 2000 K, showing its potential as a refractory material. In addition, due to its unique atomic configuration, penta-MN2 exhibits intriguing electronic properties. Penta-PdN2 is metallic, while penta-PtN2 is semiconducting with a direct band gap of 75 meV and an ultrahigh carrier mobility. This study expands the family of 2D metal nitrides, and provides new insight into finding new materials using a reliable structure search algorithm rather than chemical and physical intuition.

The hardness mechanism and bonding properties of CrN2: A first principle study

The hexagonal-close-packed (hcp) chromium dinitride (CrN2) with high hardness of 46 GPa has been selected as a model system to study the debatable hardness mechanism of AB2 type transition metal nitrides. The stress-strain relationship of hcp-CrN2 under different tensile and shear directions has been extensively studied. Theoretical shear simulation results indicate that the (101¯0) 〈1¯21¯0〉 slip direction of shear mode dominates the plastic deformation of hcp-CrN2, the ideal shear strength is only 30 GPa when the structural instability occurs, demonstrating that the hcp-CrN2 compound is not a superhard material but is indeed a hard material. Furthermore, the electronic calculations show the properties of semiconductors for hcp-CrN2, the atomic deformation mode of hcp-CrN2 indicates that the gradual separation of N2 dimers in the lattice is the main reasons for the decrease of hardness. Besides, our calculation results provide a reference value to judge transition metal nitrides whether can be a superhard material.

Penta-AlN2 monolayer: A ferromagnetic insulator

Abstract A ferromagnetic insulating behavior is necessary to realize quantum anomalous Hall effect. Here we study, using first-principles calculations and a Monte Carlo simulation, the electronic structure and magnetism of AlN2 monolayer. Our results show that the AlN2 monolayer has a ferromagnetic insulating behavior. By using the crystal field analysis, we find that the axes of the neighboring N2 dimers are perpendicular, and the superexchange between them accounts for the ferromagnetic (FM) order in the AlN2 monolayer. We also carried out a Monte Carlo simulation, which shows that the AlN2 monolayer remains FM with Curie temperature (Tc) ∼ 22 K. Moreover, the tensile strain would make a stronger overlap between the neighboring N2 dimers, and steadily enhances the FM stability. As a result, the Tc can be increased with the tensile strain.

Intercalation of alkali metals (Li, Na, and K) in molybdenum dinitride (MoN2) and titanium dinitride (TiN2) from first-principles calculations

We studied ternaries of nitrogen-rich titanium and molybdenum compounds combined with alkali metals (Li, Na, and K) as potential layered materials. LiMoN2 has already been synthesized with layered structure corresponding to intercalated 3R-MoS2, however efforts to completely deintercalate Li from LiMoN2 were unsuccessful. We studied ternaries MAN2 (A: Ti, Mo and M: Li, Na, K) having layered crystal structures, their deintercalation, and the layered binaries TiN2 and MoN2 within density-functional theory. We find that LiMoN2 and NaMoN2 have a layered structure isotypic to 2H-MoS2 with Li and Na ions filling interlayer spaces whereas LiTiN2 and NaTiN2 are isotypic to α-NaFeO2. Both KMoN2 and KTiN2 have a different kind of structure isotypic to SrTiN2 which differentiates K from Li and Na. Ternaries Li(Na)MoN2 and Li(Na)TiN2 are all metals and the alkali metal atoms are present as ions in these structures. Partially deintercalated ternaries suggest that layers can interact strongly and the material can loose its layered form. Furthermore, we find that binary MoN2 having a layered structure is not stable since monolayer MoN2 has a positive formation energy and N atoms belonging to neighboring layers interact and form N2 dimer in between Mo layers in which case formation energy becomes negative indicating that structure becomes more stable. These results can explain the decomposition of LiMoN2 during the experimental trials of complete deintercalation. In contrast, TiN2 has a negative formation energy already without interaction of N atoms belonging to neighboring layers.

Direct Reaction of Nitrogen and Lithium up to 75 GPa: Synthesis of the Li3N, LiN, LiN2, and LiN5 Compounds.

A wide variety of Li-N compounds are predicted as stable under pressure and associated with various nitrogen anionic moieties. Accordingly, the LiN5 compound was recently synthesized at 45 GPa by the direct reaction of nitrogen and lithium. In this study, we present an experimental investigation of the Li-N binary phase diagram from ambient pressure up to 73.6 GPa. The samples loaded in the diamond anvil cells were constituted of pure lithium pieces embedded in a much greater quantity of molecular nitrogen and, at incremental pressure steps, were laser-heated to produce the thermodynamically favored solid. The following compounds are observed: Li3N, LiN2, LiN as well as LiN5, and their pressure stability domain is disclosed. Two are synthesized for the first time, namely Cmcm LiN and P63/ mmc LiN2. Both are structurally resolved and characterized by X-ray diffraction and Raman spectroscopy measurements. Their high bulk modulus is characteristic of charged N2 dimers.

Insights into Antibonding Induced Energy Density Enhancement and Exotic Electronic Properties for Germanium Nitrides at Modest Pressures

Here, the electronic and bonding features in ground-state structures of germanium nitrides under different components that not accessible at ambient conditions have been systematically studied. The forming essence of weak covalent bonds between the Ge and N atom in high-pressure ionic crystal Fd-3 m-Ge3N4 is induced by the binding effect of electronic clouds originated from the Ge_ p orbitals. Hence, it helps us to understand the essence of covalent bond under high pressure, profoundly. As an excellent reducing agent, germanium transfer electrons to the antibonding state of the N2 dimer in Pa-3-GeN2 phase at 20 GPa, abnormally, weakening the bonding strength considerably than nitrogen gap (N≡N) at ambient pressure. Furthermore, the common cognition that the atomic distance will be shortened under the high pressures has been broken. Amazingly, with a lower range of synthetic pressure (∼15 GPa) and nitrogen contents (28%), its energy density is up to 2.32 kJ·g-1, with a similar order of magnitude than polymeric LiN5 (nonmolecular compound, 2.72 kJ·g-1). It breaks the universal recognition once again that nitrides just containing polymeric nitrogen were regarded as high energy density materials. Hence, antibonding induced energy density enhancement mechanism for low nitrogen content and pressure has been exposed in view of electrons. Both the highest occupied molecular orbitals (HOMO) and the lowest unoccupied molecular orbitals (LUMO) are usually the separated orbitals of N_π* and N_σ*, which are the key to stabilization. Besides, the sp2 hybridizations that exist in N4 units are responsible for the stability of the R-3 c-GeN4 structure and restrict the delocalization of electrons, exhibiting nonmetallic properties.

XANES study of vanadium and nitrogen dopants in photocatalytic TiO2 thin films.

We report an X-ray absorption near edge structure (XANES) study of vanadium (V) and nitrogen (N) dopants in anatase TiO2 thin films deposited by radio-frequency magnetron sputtering. Measurements at the Ti K and V K edges were combined with soft X-ray experiments at the Ti L2,3, O K and N K edges. Full potential ab initio spectral simulations of the V, O and N K-edges were carried out for different possible configurations of substitutional and interstitial dopant-related point defects in the anatase structure. The comparison between experiments and simulations demonstrates that V occupies substitutional cationic sites (replacing Ti) irrespective of the film structure and dopant concentration (up to 4.5 at%). On the other hand, N is found both in substitutional anionic sites (replacing O) and as N2 dimers within TiO2 interstices. The dopants' local structures are discussed with reference to the enhanced optical absorption and photocatalytic activity achieved by (co)doping.

Benchmark of Dynamic Electron Correlation Models for Seniority-Zero Wave Functions and Their Application to Thermochemistry.

Wave functions restricted to electron-pair states are promising models to describe static/nondynamic electron correlation effects encountered, for instance, in bond-dissociation processes and transition-metal and actinide chemistry. To reach spectroscopic accuracy, however, the missing dynamic electron correlation effects that cannot be described by electron-pair states need to be included a posteriori. In this Article, we extend the previously presented perturbation theory models with an Antisymmetric Product of 1-reference orbital Geminal (AP1roG) reference function that allows us to describe both static/nondynamic and dynamic electron correlation effects. Specifically, our perturbation theory models combine a diagonal and off-diagonal zero-order Hamiltonian, a single-reference and multireference dual state, and different excitation operators used to construct the projection manifold. We benchmark all proposed models as well as an a posteriori Linearized Coupled Cluster correction on top of AP1roG against CR-CC(2,3) reference data for reaction energies of several closed-shell molecules that are extrapolated to the basis set limit. Moreover, we test the performance of our new methods for multiple bond breaking processes in the homonuclear N2, C2, and F2 dimers as well as the heteronuclear BN, CO, and CN+ dimers against MRCI-SD, MRCI-SD+Q, and CR-CC(2,3) reference data. Our numerical results indicate that the best performance is obtained from a Linearized Coupled Cluster correction as well as second-order perturbation theory corrections employing a diagonal and off-diagonal zero-order Hamiltonian and a single-determinant dual state. These dynamic corrections on top of AP1roG provide substantial improvements for binding energies and spectroscopic properties obtained with the AP1roG approach, while allowing us to approach chemical accuracy for reaction energies involving closed-shell species.

Ground-State Structure of YN2 Monolayer Identified by Global Search

Transition metal dinitrides (TMDNs) have attracted increasing attention for their rich chemistry, intriguing properties, and potential applications in electronic devices and electrodes. The similarity in atomic ratio with transition-metal dichalcogenide (TMD) sheets leads to an assumption in previous studies that TMDN sheets adopt similar geometry to that of TMD sheets. Here, using global particle-swarm optimization method combined with first-principles calculations, we show a distinct structure of YN2 monolayer containing isolated N2 dimers labeled as O-YN2, which is dynamically, thermally and mechanically stable, and energetically favorable over the previously predicted H- and T-YN2 monolayer structures. Moreover, because of its unique atomic configuration, the O-YN2 sheet is metallic, providing an intrinsic advantage in electrical conductivity over those semiconducting or insulating transition-metal oxides and TMD layers. In particular, we find that O-YN2 is a promising anode material for potassium ion...

Improved parameterization of the quantum harmonic oscillator model based on localized wannier functions to describe Van der Waals interactions in density functional theory

Recently, the quantum harmonic oscillator model has been combined with maximally localized Wannier functions to account for long‐range dispersion interactions in density functional theory calculations (Silvestrelli, J. Chem. Phys. 2013, 139, 054106). Here, we present a new, improved set of values for the three parameters involved in this scheme. To test the new parameter set we have computed the potential energy curves for various systems, including an isolated Ar2 dimer, two N2 dimers interacting within different configurations, and a water molecule physisorbed on pristine graphene. While the original set of parameters generally overestimates the interaction energies and underestimates the equilibrium distances, the new parameterization substantially improves the agreement with experimental and theoretical reference values. © 2016 Wiley Periodicals, Inc.