Electronic Population Research Articles

Rapid screening of alloy elements to improve the elastic properties of dilute Pt-based alloys: High-throughput first-principles calculations and modeling

Platinum (Pt)-based dilute solid solutions are an important category of high-temperature alloys and bond coatings. In this study, the effects of 33 alloying elements on the mechanical and electronic properties of dilute Pt-based solid solutions are systematically investigated under atom relaxation and full relaxation using first-principles calculations based on density functional theory. The negative mixing enthalpy of Pt-dilute solid solutions means that the solubility of the solute elements in the Pt-based dilute alloys is energetically favorable at 0 K. Niobium, rhenium, and scandium are promising candidate elements for increasing the hardness and ductility of dilute Pt-based solid solutions. In addition, the electronic basis for the mechanical properties of Pt-dilute solid solutions is investigated in terms of the electronic density and mean bond population. The results demonstrate that the Pt–X bond lengths are shorter than the Pt–Pt bond length, resulting in greater hardness. Moreover, the model for the composition dependent elastic properties is built based on the CALPHAD approach, which will be used to the Pt-based multiphase alloys in the future. As certain alloying elements improve the hardness and ductility of Pt, this research expands our knowledge of the mechanism of dilute Pt-based solid solutions and provides a basis for next-generation superalloys or bond coatings at higher temperatures.

Superfluid density of a photo-induced superconducting state

Nonequilibrium conditions offer novel routes to superconductivity that are not available at equilibrium. For example, by engineering nonequilibrium electronic populations, pairing may develop between electrons in different energy bands. A concrete proposal has been made to photo-induce superconductivity in a semiconductor, where pairing occurs between electrons in the conduction and valence bands, even for repulsive interactions. Here, we calculate the superfluid density for such a nonequilibrium paired state, and find it to be positive for repulsive interactions and interband pairing. The positivity of the superfluid density implies the stability of the photo-induced superconducting state as well as the existence of the Meissner effect.

Current-induced dissociation in molecular junctions beyond the paradigm of vibrational heating: The role of antibonding electronic states

The interaction between electronic and nuclear degrees of freedom in single-molecule junctions is an essential mechanism, which may result in the current-induced rupture of chemical bonds. As such, it is fundamental for the stability of molecular junctions and for the applicability of molecular electronic devices. In this publication, we study current-induced bond rupture in molecular junctions using a numerically exact scheme, which is based on the hierarchical quantum master equation (HQME) method in combination with a discrete variable representation for the nuclear degrees of freedom. Employing generic models for molecular junctions with dissociative nuclear potentials, we identify distinct mechanisms leading to dissociation, namely the electronic population of anti-bonding electronic states and the current-induced heating of vibrational modes. Our results reveal that the latter plays a negligible role whenever the electronic population of anti-bonding states is energetically possible. Consequently, the significance of current-induced heating as a source for dissociation in molecular junctions involving an active anti-bonding state is restricted to the non-resonant transport regime, which reframes the predominant paradigm in the field of molecular electronics.

Electronic structure of graphene nanoribbons under external electric field by density functional tight binding

<sec> In recent years, the rapid development of electronic information technology has brought tremendous convenience to people’s lives, and the devices used have become increasingly miniaturized. However, due to the constraints of the process and the material itself, as the size of the devices made of silicon materials is further reduced, obvious short channel effects and dielectric tunneling effects will appear, which will affect the normal operations of these devices. In order to overcome this development bottleneck, it is urgent to find new materials for the devices that can replace silicon. Carbon has the same outer valence electron structure as silicon. Since 2004, Geim [Novoselov K S, Geim A K, Morozov S V, et al. 2005 <i>Nature</i> <b>438</b> 197] prepared two-dimensional graphene with a honeycomb-like planar structure formed by sp<sup>2</sup> hybridization, graphene has received extensive attention from researchers and industrial circles for its excellent electronic and mechanical properties. However, graphene is not a true semiconductor, and it has no band gap in its natural state. The energy gap can be opened by preparing graphene nanoribbons. On this basis, the electronic structure of the nanoribbons can be further controlled by using an external electric field to destroy the symmetric structure of the nanoribbons. </sec><sec>In this paper, the tight-binding method based on density functional theory is used to calculate and study the influence of external transverse electric field on the electronic structure and electron population of un-hydrogenated/hydrogenated armchair graphene nanoribbons. The calculation results show that whether there is hydrogen on the edge of the graphene nanoribbons or not, the energy gap changed at the Г point shows a three-group periodic oscillation decreasing law, and as N increases, the energy gap will disappear. Under the external electric field, the band structure and the density of states of the nanoribbons will change greatly. For un-hydrogenated nanoribbons with semiconducting properties, as the intensity of the external electric field increases, a semiconductor-metal transition occurs. At the same time, the electric field will also have a significant influence on the energy level distribution, resulting in significant changes in the peak height and peak position of the density of states. The external electric field causes the electron population distribution on the atoms in the nanoribbons to break its symmetry. The greater the electric field strength, the more obvious the population asymmetry is. The edge hydrogenation passivation can significantly change the population distribution of atoms in nanoribbons.</sec>

Electronic structure basis of strength and toughness in fluoropolymers

Using density functional theory calculations, we investigate and report the electronic structure basis of symmetry-breaking in atomic structures and variations in extreme mechanical properties of a number of widely used polymers, namely, polyvinylidene fluoride (PVDF), polytetrafluoroethylene, polyethylene, and three structural variants of PVDF. All of these polymers consist of C–C bonds in the backbone and a combination of C–H and C–F bonds attached to the backbone. Our results show that the relative proportion and arrangement of the C–H and C–F bonds, hereafter called the chemical heterogeneity, lead to two distinct sets of structural symmetries and mechanical properties. The set with the lower chemical heterogeneity possesses higher structural symmetry and exhibits higher strength and toughness. Electronic population analysis shows that chemical heterogeneity breaks the structural symmetry and alters deformation induced “electron redistribution patterns” in the C–C backbone. Strong variations in electron redistribution serve as the basis for distinctive mechanical behavior among the polymers: the higher the structural variation and electron redistribution, the lower the bond rupture force and toughness of the polymer. The bond rupture process is initiated by a decrease in the electronic population in the backbone, while the electronic interactions at the C–H and C–F bonds dictate where, how, and which bonds rupture. Furthermore, in the linear regime of mechanical deformation, the force–strain behavior is insensitive to the chemical heterogeneity and controlled by the strong covalent interactions of the C–C bonds. In the nonlinear regime, the chemical heterogeneity dramatically reduces the strength and toughness of the polymer chain.

Enhancing the dynamic electron transfer of Au species on wormhole-like TS-1 for boosting propene epoxidation performance with H2 and O2

Engineering unique electronic structure of catalyst to boost catalytic performance is of prime scientific and industrial importance. Herein, the identification of intrinsic electronic sensitivity for direct propene epoxidation was first achieved over highly stable Au/wormhole-like TS-1 catalyst. Results show that the electron transfer of Au species can be regulated by manipulating the dynamic evolutions and contents of Au valence states, thus resulting in different catalytic performance in 100 h time-on-stream. By DFT calculations, kinetic analysis and multi-characterizations, it is found that the Au0 species with higher electronic population can easily transfer more electrons to activate surface O2 compared with Au1+ and Au3+ species. Moreover, there is a positive correlation between Au0 content and activity. Based on this correlation, a facile strategy is further proposed to boost Au0 percentage, resulting in the reported highest PO formation rate without adding promoters. This work harbors tremendous guiding significance to the design of highly efficient Au/Ti-containing catalyst for propene epoxidation with H2 and O2.

Interatomic Coulombic decay of a Li dimer in a coupled electron and nuclear dynamics approach

Interatomic Coulombic decay (ICD) is a fundamental process between atoms or molecules via a neighbor interaction that produces a relaxation of an electronically excited atom or molecule when embedded in an environment. Due to the physical nature of the process, the electronic and nuclear degrees of freedom are coupled. In this paper, we study the ICD process for a lithium dimer by means of the electron and nuclear dynamics (END) approach. The END approach incorporates a full time-dependent description of the electronic and nuclear degrees of freedom, although its current version does not properly account for continuum states and has limitations in the electronic description by using a single determinantal wave function. Despite this, we confirm that the ICD process takes place via a dipole interaction that induces the nuclear motion of the dimer with a consequent electronic population transfer to higher excited states simulating the ionization process. When the dimer approaches a distance of around 11.5 a.u. (6 \AA{}), this ionization process takes place due to the dipole coupling and occurs at the place of the strongest attractive dipole force. Passing that point, we find that the two lithium atoms repel each other via a Coulomb explosion process followed with a consequent kinetic-energy release (KER). We determine the KER and the timing of the ICD process. We point out the strengths and weaknesses of the END approach and the required enhancements to account for a better description of the ICD process in a coupled electron and nuclear dynamics.

Formation of excited triplet states in naphthalene diimide and perylene diimide derivatives: A detailed theoretical study.

Mechanistic details of the excited triplet state formation upon photoexcitation to the low-lying singlet manifold in naphthalene diimide and perylene diimide derivatives are explored theoretically. Static and dynamic aspects of two singlets (S1 and S2) and six triplets (T1-T6) of these molecules are investigated. Suitable vibronic Hamiltonians are constructed to investigate the internal conversion dynamics in both the singlet and triplet manifolds. Computed singlet-triplet energetics, spin-orbit coupling matrix elements, and intersystem crossing rates strongly suggest an efficient intersystem crossing process involving higher triplet states (T6, T5, and T4). Separate full dimensional quantum wavepacket simulations of singlet and triplet manifolds in the approximate linear vibronic model by assuming initial Franck-Condon conditions are carried out to unravel the internal conversion decay dynamics in the respective manifolds. The obtained diabatic electronic populations and nuclear densities are analyzed to illustrate the triplet generation pathways involving higher triplet states in these molecules.

Quantum-optical realization of an Ornstein-Uhlenbeck-type process via simultaneous action of white noise and feedback

We establish an important connection between coherent quantum feedback and the Ornstein-Uhlenbeck process in quantum optics. We show that an emitter with fluctuating energy levels in front of a mirror results in an Ornstein-Uhlenbeck process for electronic populations, although the fluctuation of the energy levels is assumed to be uncorrelated in time and space. Based on a Heisenberg equation of motion description of the quantum feedback dynamics, we discuss additionally the impact of phase noise on the population dynamics and provide examples in which noise itself is not detrimental but supports and enhances typical features of quantum feedback such as self-stabilization.

Ab Initio Modeling of the Electronic and Energy Structure and Opening the Band Gap of a 4p-Element-Doped Graphene Monolayer

In this paper, we study the electronic and band structure of 4p-elements (M = Ge, Si)-doped graphene nanosheets with vacancies by the density functional theory method. The adsorption energy of the doping atoms and the relative stability of doped graphene monolayers are estimated. An antiferromagnetic ordering is discovered in these graphene-based systems. Based on the analysis of the electronic populations of atomic orbitals according to Mulliken, the states’ densities in graphene systems are calculated. The equilibrium parameters of the electronic structure of graphene nanosheets are obtained. The regularities of changes in the electronic structure of the valence band and the induction of an energy gap in M-doped graphene monolayers containing vacancies are studied. The features of the electronic structure near the Fermi level, as well as the role of the structural effect in opening the energy gap in graphene–M systems, are discussed. Doping with 4p elements opens the energy gap in graphene–Ge(Si) systems. The local magnetic moments for antiferromagnetic ordering are estimated on carbon atoms of graphene nanosheets including Ge(Si). The calculated values ​​of the local magnetic moments on carbon atoms in graphene–M (Ge, Si) systems are comparable with each other.

Modeling and simulation of the adsorption and storage of hydrogen in calcite rock oil fields.

Due to the thermodynamic conditions prevailing at very shallow depths of calcite stone oil fields, molecular hydrogen has been reported to be released from hydrocarbon or heavy oil located on the surface of the calcite stone. Since this region is physically inaccessible, there is a need to realize modeling and simulation of the hydrogen adsorption and storage process under reservoir conditions. Motivated by the previous problem, in this work, based on recent reports of hydrogen production from oil fields, we present a theoretical methodology to describe the process of hydrogen adsorption on naturally fractured and carbonated (limestone (CaCO3)) reservoirs and to quantify their storage capacity. Firstly, the calcite rock model was optimized inside a simulation cell containing a vacuum layer, for which energy optimization techniques based on density functional theory were used. Subsequently, using ab initio methods also based on DFT, calcite rock was characterized obtaining structural, electronic, vibrational, thermodynamic properties, and Mulliken population analysis of CaCO3. Finally, molecular dynamics simulations were performed in order to simulate the adsorption process and obtain percentages of hydrogen adsorption on (110) surface of the (2 × 2) CaCO3 supercell, for N = 3, 5, 10 hydrogen molecules. The molecular dynamics simulations showed that the surface of CaCO3 rock has hydrogen capacity of only 0.42 mass %.

Electronic Population Transfer via Impulsive Stimulated X-Ray Raman Scattering with Attosecond Soft-X-Ray Pulses.

Free-electron lasers provide a source of x-ray pulses short enough and intense enough to drive nonlinearities in molecular systems. Impulsive interactions driven by these x-ray pulses provide a way to create and probe valence electron motions with high temporal and spatial resolution. Observing these electronic motions is crucial to understand the role of electronic coherence in chemical processes. A simple nonlinear technique for probing electronic motion, impulsive stimulated x-ray Raman scattering (ISXRS), involves a single impulsive interaction to produce a coherent superposition of electronic states. We demonstrate electronic population transfer via ISXRS using broad bandwidth (5.5eV full width at half maximum) attosecond x-ray pulses produced by the Linac Coherent Light Source. The impulsive excitation is resonantly enhanced by the oxygen 1s→2π^{*} resonance of nitric oxide (NO), and excited state neutral molecules are probed with a time-delayed UV laser pulse.

Effects of intramolecular vibrations on excitation energy transfer dynamics of the Fenna-Matthews-Olson complex

The role of vibrational coherence in photosynthetic pigment-proteins has been controversial. The present article investigates theoretically how the intramolecular vibrations of bacteriochlorophyll (BChl) influence the excitation energy transfer (EET) dynamics in the Fenna-Matthews-Olson complex (FMO). To this end, we use an exciton model incorporating recent experimental data and treat the vibrations as both coherent and incoherent. We simulate the transfer phenomena numerically with non-Markovian quantum dynamics. Our results at 300 K indicate that incoherent vibrations can moderately improve the efficiency of EET between FMO and a reaction center. With a vibronic model, we show that the coherent vibrations in either strongly or resonantly coupled BChl pairs can generate characteristic oscillations in their electronic populations which persist for several picoseconds even at 300 K. Despite such robust coherences, no significant improvement in the efficiency of EET is observed.

Pressure-induced the formation of Mg(CH3)2 and Ca(CH3)2 studied by the first principles

Hydrogen-rich materials have always attracted the interest for the crystal structures and electronic properties under high pressures. The crystal structures of alkaline earth compounds Mg(CH3)2 and Ca(CH3)2 have been predicted by employing a particle swarm optimization algorithm at high pressures. The P-3c1 phases of the both compounds were synthesized at about 17.5 GPa and 3 GPa, respectively. The current thermodynamical calculations reveal that the P-3c1 phases are the most stable up to 50 GPa. No imaginary phonon frequencies in the whole Brillouin zone indicate the P-3c1stuctures of Mg(CH3)2 and Ca(CH3)2 are dynamically stable. The computations of the elastic constants indicate P-3c1 structures satisfy the mechanical stability criteria under high pressures. The strong covalent bonds between C atoms and H atoms have been proved by the electronic properties and population analysis, which confirms that the CH3 groups can exist stable.

Diels-Alder reaction mechanisms of substituted chiral anthracene: A theoretical study based on the reaction force and reaction electronic flux.

Quantum chemical calculations were used to study the mechanism of Diels-Alder reactions involving chiral anthracenes as dienes and a series of dienophiles. The reaction force analysis was employed to obtain a detailed scrutiny of the reaction mechanisms, it has been found that thermodynamics and kinetics of the reactions are quite consistent: the lower the activation energy, the lower the reaction energy, thus following the Bell-Evans-Polanyi principle. It has been found that activation energies are mostly due to structural rearrangements that in most cases represented more than 70% of the activation energy. Electronic activity mostly due to changes in σ and π bonding were revealed by the reaction electronic flux (REF), this property helps identify whether changes on σ or π bonding drive the reaction. Additionally, new global indexes describing the behavior of the electronic activity were introduced and then used to classify the reactions in terms of the spontaneity of their electronic activity. Local natural bond order electronic population analysis was used to check consistency with global REF through the characterization of specific changes in the electronic density that might be responsible for the activity already detected by the REF. Results show that reactions involving acetoxy lactones are driven by spontaneous electronic activity coming from bond forming/strengthening processes; in the case of maleic anhydrides and maleimides it appears that both spontaneous and non-spontaneous electronic activity are quite active in driving the reactions.

The impact of electron–electron correlation in ultrafast attosecond single ionization dynamics

The attosecond ultrafast ionization dynamics of correlated two- or many-electron systems have, so far, been mainly addressed investigating atomic systems. In the case of single ionization, it is well known that electron–electron correlation modifies the ionization dynamics and observables beyond the single active electron picture, resulting in effects such as the Auger effect or shake-up/down and knock-up/down processes. Here, we extend these works by investigating the attosecond ionization of a molecular system involving correlated two-electron dynamics, as well as non-adiabatic nuclear dynamics. Employing a charge-transfer molecular model system with two differently bound electrons, a strongly and a weakly bound electron, we distinguish different pathways leading to ionization, be it direct ionization or ionization involving elastic and inelastic electron scattering processes. We find that different pathways result in a difference in the electronic population of the parent molecular ion, which, in turn, involves different subsequent (non-adiabatic) postionization dynamics on different time scales.

Supervised Bayesian Statistical Learning to Identify Prognostic Risk Factor Patterns from Population Data.

Current methods for building risk models assume averaged uniform effects across populations. They use weighted sums of individual risk factors from regression models with only a few interactions, such as age. This does not allow risk factor effects to vary in different morbidity contexts. This study modified a supervised Bayesian statistical learning method of topic modelling, allowing individual factors to have different effects depending on a patient's other comorbidity. This study used topic modelling to assess more than 71,000 unique risk factors in a population cohort of 1.4 million adults within routine data. The model learnt prognostically important risk factor patterns that predicted 5 year survival, and the resulting model achieved excellent calibration and discrimination with a C statistic of 0.9 in a held out validation cohort. The model explained 92% of the observed variation in 5 year survival in the population. This paper validates using survival supervised Bayesian topic modelling within large routine electronic population health data to identify prognostically important risk factor patterns.

Second Sphere Ligand Promoted Organoiridium Catalysts for Methanol Dehydrogenation under Mild Conditions

AbstractHydrogen produced from renewable resources is a potential source of clean energy. And Methanol is a liquid at room temperature and has a hydrogen content of up to 12.6 wt%, makes it a promising hydrogen source and can provide great benefits for sustainable hydrogen production. In this work, the modification of the picolinic acid ligand along with changes in the type and substituent position of the groups enhances the activity in iridium‐catalyzed methanol dehydrogenation catalysis. These second sphere ligand modifications are found to be effective for achieving highly active organoiridium catalysts for methanol dehydrogenation reaction under mild conditions, i. e. 1 atm, 78 °C, with a weak base and without using an organic solvent, exhibiting a TOF of 377 h−1. The electronic population analysis of each ligand is theoretically calculated, which shows that catalytic activity is dramatically affected by modifications to the metal's secondary coordination sphere environment, and the high activity originates from appropriate coordination by the picolinic acid ligand and carboxylic acid moiety in addition to the hydroxyl functionality. Moreover, kinetic experiments suggest that the C−H bond of methanol is linked to the iridium core and broken in the rate‐determining step of the reaction. This study will be valuable for catalyst development in the field of methanol dehydrogenation.

Incorporating Electronic Information into Machine Learning Potential Energy Surfaces via Approaching the Ground-State Electronic Energy as a Function of Atom-Based Electronic Populations

Machine learning (ML) approximations to density functional theory (DFT) potential energy surfaces (PESs) are showing great promise for reducing the computational cost of accurate molecular simulations, but at present, they are not applicable to varying electronic states, and in particular, they are not well suited for molecular systems in which the local electronic structure is sensitive to the medium to long-range electronic environment. With this issue as the focal point, we present a new machine learning approach called "BpopNN" for obtaining efficient approximations to DFT PESs. Conceptually, the methodology is based on approaching the true DFT energy as a function of electron populations on atoms; in practice, this is realized with available density functionals and constrained DFT (CDFT). The new approach creates approximations to this function with neural networks. These approximations thereby incorporate electronic information naturally into a ML approach, and optimizing the model energy with respect to populations allows the electronic terms to self-consistently adapt to the environment, as in DFT. We confirm the effectiveness of this approach with a variety of calculations on LinHn clusters.

Subsystems of many-electron system and reduced density matrices

Dividing a system into subsystems is a widely used approach that allows one to calculate the electronic structure of large and complex systems. Quite often, the first order reduced density matrix of the system is employed in this approach. Unfortunately, it turns out that the obtained values of the electronic populations of subsystems, which must correspond to the number of electrons in the subsystem, are fractional and they noticeably deviate from integers. In the present paper for a system in the state of a particular type it is shown that if the second order reduced density matrix is also taken into account in the subsystem generations, then the orthogonal one-electron basis can be found with which the calculated populations of subsystems will be practically equal to integer numbers. The said state of a particular type is the state whose wave function is a single determinant with doubly occupied orbitals. This is a reasonable approximation to the wave function for the singlet ground state of a standard atomic-molecular system.