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Valleytronics is a promising paradigm to explore the emergent degree of freedom for charge carriers on the energy band edges. Using ab initio calculations, we reveal that the honeycomb boron nitride (h-BN) monolayer shows a pair of inequivalent valleys in the vicinities of the vertices of hexagonal Brillouin zone even without the protection of the C3 symmetry. The inequivalent valleys give rise to a 2-fold degree of freedom named the valley pseudospin. The valley pseudospin with a tunable bandgap from deep ultraviolet to far-infrared spectra can be obtained by doping h-BN monolayer with carbon atoms. For a low-concentration carbon periodically doped h-BN monolayer, the subbands with constant valley Hall conductance are predicted due to the interaction between the artificial superlattice and valleys. In addition, the valley pseudospin can be manipulated by visible light for high-concentration carbon doped h-BN monolayer. In agreement with our calculations, the circularly polarized photoluminescence spectra of the B0.92NC2.44 sample show a maximum valley-contrasting circular polarization of 40% and 70% at room temperature and 77 K, respectively. Our work demonstrates a class of valleytronic materials with a controllable bandgap. [ABSTRACT FROM AUTHOR]

Recent experimental breakthrough demonstrated a powerful synthesis approach for intercalating the van der Waals gap of layered materials to achieve property modulation, thereby opening an avenue for exploring new physics and devising novel applications, but the mechanism governing intercalant assembly patterns and properties remains unclear. Based on extensive structural search and energetics analysis by ab initio calculations, we reveal a Sabatier-like principle that dictates spatial arrangement of self-intercalated atoms in transition metal dichalcogenides. We further construct a robust descriptor quantifying that strong intercalant-host interactions favor a monodispersing phase of intercalated atoms that may exhibit ferromagnetism, while weak interactions lead to a trimer phase with attenuated or quenched magnetism, which further evolves into tetramer and hexagonal phases at increasing intercalant density. These findings elucidate the mechanism underpinning experimental observations and paves the way for rational design and precise control of self-intercalation in layered materials.

The atom sets an ultimate scaling limit to Moore's law in the electronics industry. While electronics research already explores atomic scales devices, photonics research still deals with devices at the micrometer scale. Here we demonstrate that photonic scaling, similar to electronics, is only limited by the atom. More precisely, we introduce an electrically controlled plasmonic switch operating at the atomic scale. The switch allows for fast and reproducible switching by means of the relocation of an individual or, at most, a few atoms in a plasmonic cavity. Depending on the location of the atom either of two distinct plasmonic cavity resonance states are supported. Experimental results show reversible digital optical switching with an extinction ratio of 9.2 dB and operation at room temperature up to MHz with femtojoule (fJ) power consumption for a single switch operation. This demonstration of an integrated quantum device allowing to control photons at the atomic level opens intriguing perspectives for a fully integrated and highly scalable chip platform, a platform where optics, electronics, and memory may be controlled at the single-atom level. [ABSTRACT FROM AUTHOR]

Ammonium adducts of trimethylsilyl-terminated poly(dimethylsiloxane) (CH-PDMS) produced by electrospray ionization were submitted to collision induced dissociation and revealed a particular MS/MS behavior: the same three main product ions at m/z 221, 295, and 369 were always generated in very similar relative abundances regardless of the size of the precursor ion. Combining accurate mass measurements and ab initio calculation allowed very stable cyclic geometries to be obtained for these ionic species. Dissociation mechanisms were proposed to account for the three targeted ions to be readily generated in a two-step or a three-step reaction from any CH-PDMS ammonium adducts. A second set of three product ions was also observed with low abundance at m/z 207, 281, and 355, which were shown in MS experiments to be formed in secondary reactions. An alternative dissociation process was shown to consist of a concerted elimination of ammonia and methane and the need for a methyl of an end-group to be involved in the released methane molecule would account for this reaction to mainly proceed from the smallest precursor ions. [ABSTRACT FROM AUTHOR]

Redox electrolytes for supercapacitors (SCs) have recently sparked widespread interest. Due to the redox reactions within electrolytes, they can achieve high capacitance and long cycle stability. However, the energy density of SCs with redox electrolytes is limited by the narrow applied electrochemical window due to the irreversible side reaction of redox mediators at high potential. To overcome this issue, a redox mediator with a high redox potential, tetrachloridehydroquinone (TCHQ), is added to organic electrolytes to obtain a broad electrochemical window. TCHQ is designed to undergo a dehydrogenation reaction catalyzed by N-doped activated carbon to provide capacitance. The pyrrole N atoms have the highest electrocatalytic activity based on the theoretical calculation of reaction overpotential with predicted reaction pathways due to their Lewis basicity. Benefitting from that, TCHQ shows promising reversibility with a larger electrochemical window (up to 2.7 V). As a result, a higher energy density is obtained when compared to commercial SCs. This study proposes a strategy for designing redox mediators and interfaces of SCs with high energy density and a calculation method of dehydrogenation reaction electrocatalysis.

The cathode material of a lithium-ion battery is a key component that affects durability, capacity, and safety. Compared to the LiCoO 2 cathode material (the reference standard for these properties), LiNiO 2 can extract more Li at the same voltage and has therefore attracted considerable attention as a material that can be used to obtain higher capacity. As a trade-off, it undergoes pyrolysis relatively easily, leading to ignition and explosion hazards, which is a challenge associated with the application of this compound. Pyrolysis has been identified as a structural phase transformation of the layered rocksalt structure → spinel → cubic rocksalt. Partial substitution of Ni with various elements can reportedly suppress the transformation and, hence, the pyrolysis. It remains unclear which elemental substitutions inhibit pyrolysis and by what mechanism, leading to costly material development that relies on empirical trial and error. In this study, we developed several possible reaction models based on existing reports, estimated the enthalpy change associated with the reaction by ab initio calculations, and identified promising elemental substitutions. The possible models were narrowed down by analyzing the correlations of the predicted dependence of the reaction enthalpies on elemental substitutions, compared between different reaction models. According to this model, substitution by P and Ta affords the highest enthalpy barrier between the initial (layered rocksalt) and the final (cubic rocksalt) structures but promotes the initial transformation to spinel as a degradation. Substitution by W instead generates the barrier to the final (preventing dangerous incidents) process, as well as for the initial degradation to spinel; therefore, it is a promising strategy to suppress the predicted pyrolysis.

The lattice structure of monolayer borophene depends sensitively on the substrate yet is metallic independent of the environment. Here, we show that bilayer borophene on Ag(111) shares the same ground state as its freestanding counterpart that becomes semiconducting with an indirect bandgap of 1.13 eV, as evidenced by an extensive structural search based on first-principles calculations. The bilayer structure is composed of two covalently bonded v 1/12 boron monolayers that are stacked in an AB mode. The interlayer bonds not only localize electronic states that are otherwise metallic in monolayer borophene but also in part decouple the whole bilayer from the substrate, resulting in a quasi-freestanding system. More relevant is that the predicted bilayer model of a global minimum agrees well with recently synthesized bilayer borophene on Ag(111) in terms of lattice constant, topography, and moiré pattern.

Two-dimensional hexagonal boron nitride (h-BN) is studied as a tunnel barrier in magnetic tunnel junctions (MTJs) as a possible alternative to MgO. The tunnel magnetoresistance (TMR) of such MTJs is calculated as a function of whether the interface involves the chemi- or physisorptive site of h-BN atoms on the metal electrodes, Fe, Co, or Ni. It is found that the physisorptive site on average produces higher TMR values, whereas the chemisorptive site has the greater binding energy but lower TMR. It is found that alloying the electrodes with an inert metal-like Pt can induce the preferred absorption site on Co to become a physisorptive site, enabling a higher TMR value. It is found that the choice of physisorptive sites of each element gives more Schottky-like dependence of their Schottky barrier heights on the metal work function.

In this work, we report a novel strategy to increase the gas adsorption selectivity of metal organic framework materials by coadsorbing another molecular species. Specifically, we find that addition of tightly bound NH 3 molecules in the well-known metal-organic framework MOF-74 dramatically alters its adsorption behavior of C 2 H 2 and C 2 H 4 . Combining in situ infrared spectroscopy and ab initio calculations, we find that-as a result of coadsorbed NH 3 molecules attaching to the open metal sites-C 2 H 2 binds more strongly and diffuses much faster than C 2 H 4 , occupying the available space adjacent to metal-bound NH 3 molecules. Most remarkably, C 2 H 4 is now almost completely excluded from entering the MOF once C 2 H 2 has been loaded. This finding dispels the widespread belief that strongly coadsorbed species in nanoporous materials always undermine their performance in adsorbing or separating weakly bound target molecules. Furthermore, it suggests a new route to tune the adsorption behavior of MOF materials through harnessing the interactions among coadsorbed guests.

Electrochemical cycling induces transition-metal (TM) ion migration and oxygen vacancy formation in layered transition-metal oxides, thus causing performance decay. Here, a combination of ab initio calculations and atomic level imaging is used to explore the TM migration mechanisms in LiNi 1/3 Mn 1/3 Co 1/3 O 2 (NMC333). For the bulk model, TM/Li exchange is an favorable energy pathway for TM migration. For the surface region with the presence of oxygen vacancies, TM condensation via substitution of Li vacancies (TM sub ) deciphers the frequently observed TM segregation phenomena in the surface region. Ni migrates much more easily in both the bulk and surface regions, highlighting the critical role of Ni in stabilizing layered cathodes. Moreover, once TM ions migrate to the Li layer, it is easier for TM ions to diffuse and form a TM-enriched surface layer. The present study provides vital insights into the potential paths to tailor layered cathodes with a high structural stability and superior performance.

Group IV alloys have been long viewed as homogeneous random solid solutions since perceiving them as Si-compatible, direct-band gap semiconductors 30 years ago. Such a perception underlies the understanding, interpretation, and prediction of alloys' properties. However, as the race to create scalable and tunable device materials enters a composition domain far beyond the alloys' equilibrium solubility, a fundamental question emerges as to how random these alloys truly are. Here, we show, by combining statistical sampling and large-scale ab initio calculations, that GeSn alloy, a promising group IV alloy for mid-infrared technology, exhibits a clear short-range order for solute atoms within its entire composition range. Such a short-range order is further found to substantially affect the electronic properties of GeSn. We demonstrate that the proper inclusion of this short-range order through canonical sampling can lead to a significant improvement over previous predictions on alloy's band gaps by showing an excellent agreement with experiments within the entire studied composition range. Our finding thus not only calls for an important revision of the current structural model for group IV alloy but also suggests that short-range order may generically exist in different types of alloys.

Single photon sources hold great promise in quantum information technologies and are often materialized by single atoms, quantum dots, and point defects in dielectric materials. Yet, these entities are vulnerable to annealing and chemical passivation, ultimately influencing the stability of photonic devices. Here, we show that topologically stable dislocations in transition metal dichalcogenide monolayers can act as single photon sources, as supported by calculated defect levels, diploe matrix elements for transition, and excitation lifetimes with first-principles. The emission from dislocations can range from 0.48 to 1.29 eV by varying their structure, charge state, and chemical makeup in contrast to the visible range provided by previously reported sources. Since recent experiments have controllably created dislocations in monolayer materials, these results open the door to utilizing robustly stable defects for quantum computing.

We present a transient absorption setup combining broadband detection over the visible-UV range with high temporal resolution (∼20 fs) which is ideally suited to trigger and detect vibrational coherences in different classes of materials. We generate and detect coherent phonons (CPs) in single-layer (1L)-MoS 2 , as a representative semiconducting 1L-transition metal dichalcogenide (TMD), where the confined dynamical interaction between excitons and phonons is unexplored. The coherent oscillatory motion of the out-of-plane A' 1 phonons, triggered by the ultrashort laser pulses, dynamically modulates the excitonic resonances on a time scale of few tens of fs. We observe an enhancement by almost 2 orders of magnitude of the CP amplitude when detected in resonance with the C exciton peak, combined with a resonant enhancement of CP generation efficiency. Ab initio calculations of the change in the 1L-MoS 2 band structure induced by the A' 1 phonon displacement confirm a strong coupling with the C exciton. The resonant behavior of the CP amplitude follows the same spectral profile of the calculated Raman susceptibility tensor. These results explain the CP generation process in 1L-TMDs and demonstrate that CP excitation in 1L-MoS 2 can be described as a Raman-like scattering process.

Two-dimensional materials, obtained by van der Waals stacking of layers, are fascinating objects of contemporary condensed matter research, exhibiting a variety of new physics. Inspired by the breakthroughs of twisted bilayer graphene (TBG), we demonstrate that twisted bilayer boron nitride (TBBN) is an even more exciting novel system that turns out to be an excellent platform to realize new correlated phases and phenomena; exploration of its electronic properties shows that in contrast to TBG in TBBN multiple families of 2,4, and 6-fold degenerate flat bands emerge without the need to fine tune close to a "magic angle", resulting in dramatic and tunable changes in optical properties and exciton physics, and providing an additional platform to study strong correlations. Upon doping, unforeseen new correlated phases of matter (insulating and superconducting) emerge. TBBN could thus provide a promising experimental platform, insensitive to small deviations in the twist angle, to study novel exciton condensate and spatial confinement physics, and correlations in two dimensions.

Two-dimensional (2D) van der Waals (vdW) materials provide the possibility of realizing heterostructures with coveted properties. Here, we report a theoretical investigation of the vdW magnetic tunnel junction (MTJ) based on VSe 2 /MoS 2 heterojunction, where the VSe 2 monolayer acts as a ferromagnet with room-temperature ferromagnetism. We propose the concept of spin-orbit torque (SOT) vdW MTJ with reliable reading and efficient writing operations. The nonequilibrium study reveals a large tunneling magnetoresistance of 846% at 300 K, identifying significantly its parallel and antiparallel states. Thanks to the strong spin Hall conductivity of MoS 2 , SOT is promising for the magnetization switching of VSe 2 free layer. Quantum-well states come into being and resonances appear in MTJ, suggesting that the voltage control can adjust transport properties effectively. The SOT vdW MTJ based on VSe 2 /MoS 2 provides desirable performance and experimental feasibility, offering new opportunities for 2D spintronics.

We study the effect of electron doping on the bonding character and stability of two-dimensional (2D) structures of elemental boron, called borophene, which is known to form many stable allotropes. Our ab initio calculations for the neutral system reveal previously unknown stable 2D ϵ-B and ω-B structures. We find that the chemical bonding characteristic in this and other boron structures is strongly affected by extra charge. Beyond a critical degree of electron doping, the most stable allotrope changes from ϵ-B to a buckled honeycomb structure. Additional electron doping, mimicking a transformation of boron to carbon, causes a gradual decrease in the degree of buckling of the honeycomb lattice that can be interpreted as piezoelectric response. Net electron doping can be achieved by placing borophene in direct contact with layered electrides such as Ca 2 N. We find that electron doping can be doubled by changing from the B/Ca 2 N bilayer to the Ca 2 N/B/Ca 2 N sandwich geometry.

We study the conversion of bulk Se and Te, consisting of intertwined a helices, to structurally very dissimilar, atomically thin two-dimensional (2D) layers of these elements. Our ab initio calculations reveal that previously unknown and unusually stable δ and η 2D allotropes may form in an intriguing multistep process that involves a concerted motion of many atoms at dislocation defects. We identify such a complex reaction path involving zipper-like motion of such dislocations that initiate structural changes. With low activation barriers ≲0.3 eV along the optimum path, the conversion process may occur at moderate temperatures. We find all one-dimensional (1D) and 2D chalcogen structures to be semiconducting.

The optical control of atomic relocations in a metallic quantum point contact is of great interest because it addresses the fundamental limit of "CMOS scaling". Here, by developing a platform for combined electronics and photonics on the atomic scale, we demonstrate an optically controlled electronic switch based on the relocation of atoms. It is shown through experiments and simulations how the interplay between electrical, optical, and light-induced thermal forces can reversibly relocate a few atoms and enable atomic photodetection with a digital electronic response, a high resistance extinction ratio (70 dB), and a low OFF-state current (10 pA) at room temperature. Additionally, the device introduced here displays an optically induced pinched hysteretic current (optical memristor). The photodetector has been tested in an experiment with real optical data at 0.5 Gbit/s, from which an eye diagram visualizing millions of detection cycles could be produced. This demonstrates the durability of the realized atomic scale devices and establishes them as alternatives to traditional photodetectors.

Amorphous carbon (a-C) films composited with transition layers exhibit the desirable improvement of adhesion strength between films and substrate, but the further understanding on the interfacial structure transformation of a-C structure induced by transition layers is still lacked. In this paper, using ab initio calculations, we comparatively studied the interfacial structure between Ti, Cr, or W transition layers and a-C film from the atomic scale, and demonstrated that the addition of Ti, Cr, or W catalyzed the graphitic transformation of a-C structure at different levels, which provided the theoretical guidance for designing a multilayer nanocomposite film for renewed application.

Inorganic/biomolecule nanohybrids can combine superior electronic and optical properties of inorganic nanostructures and biomolecules for optoelectronics with performance far surpassing that achievable in conventional materials. The key toward a high-performance inorganic/biomolecule nanohybrid is to design their interface based on the electronic structures of the constituents. A major challenge is the lack of knowledge of most biomolecules due to their complex structures and composition. Here, we first calculated the electronic structure and optical properties of one of the cytochrome c (Cyt c) macromolecules (PDB ID: 1HRC ) using ab initio OLCAO method, which was followed by experimental confirmation using ultraviolet photoemission spectroscopy. For the first time, the highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels of Cyt c, a well-known electron transport chain in biological systems, were obtained. On the basis of the result, pairing the Cyt c with semiconductor single-wall carbon nanotubes (s-SWCNT) was predicted to have a favorable band alignment and built-in electrical field for exciton dissociation and charge transfer across the s-SWCNT/Cyt c heterojunction interface. Excitingly, photodetectors based on the s-SWCNT/Cyt c heterojunction nanohybrids demonstrated extraordinary ultra-broadband (visible light to infrared) responsivity (46-188 A W -1 ) and figure-of-merit detectivity D* (1-6 × 10 10 cm Hz 1/2 W -1 ). Moreover, these devices can be fabricated on transparent flexible substrates by a low-lost nonvacuum method and are stable in air. These results suggest that the s-SWCNT/biomolecule nanohybrids may be promising for the development of CNT-based ultra-broadband photodetectors.

Valleytronics is a promising paradigm to explore the emergent degree of freedom for charge carriers on the energy band edges. Using ab initio calculations, we reveal that the honeycomb boron nitride (h-BN) monolayer shows a pair of inequivalent valleys in the vicinities of the vertices of hexagonal Brillouin zone even without the protection of the C 3 symmetry. The inequivalent valleys give rise to a 2-fold degree of freedom named the valley pseudospin. The valley pseudospin with a tunable bandgap from deep ultraviolet to far-infrared spectra can be obtained by doping h-BN monolayer with carbon atoms. For a low-concentration carbon periodically doped h-BN monolayer, the subbands with constant valley Hall conductance are predicted due to the interaction between the artificial superlattice and valleys. In addition, the valley pseudospin can be manipulated by visible light for high-concentration carbon doped h-BN monolayer. In agreement with our calculations, the circularly polarized photoluminescence spectra of the B 0.92 NC 2.44 sample show a maximum valley-contrasting circular polarization of 40% and 70% at room temperature and 77 K, respectively. Our work demonstrates a class of valleytronic materials with a controllable bandgap.

Amorphous carbon films (a-C) codoped by two metal elements exhibit the desirable combination of tribological and mechanical properties for widely potential applications, but are also prone to catastrophic failure due to the inevitable residual compressive stress. Thus far, the residual stress reduction mechanism remains unclear due to the insufficient understanding of the structure from the atomic and electronic scale. In this paper, using ab initio calculations, we first designed a novel Cu/Cr codoped a-C film and demonstrated that compared with pure and Cu/Cr monodoped cases, the residual stress in Cu/Cr codoped a-C films could be reduced by 93.6% remarkably. Atomic bond structure analysis revealed that the addition of Cu and Cr impurities in amorphous carbon structure resulted in the critical and significant relaxation of distorted C-C bond lengths. On the other hand, electronic structure calculation indicated a weak bonding interaction between the Cr and C atoms, while the antibonding interaction was observed for the Cu-C bonds, which would play a pivot site for the release of strain energy. Those interactions combined with the structural evolution could account for the drastic residual stress reduction caused by Cu/Cr codoping. Our results provide the theoretical guidance and desirable strategy to design and fabricate a new nanocomposite a-C films with combined properties for renewed applications.

Oxygen intercalation has been proven to be an efficient experimental approach to decouple chemical vapor deposition grown graphene from metal substrate with mild damage, thereby enabling graphene transfer. However, the mechanism of oxygen intercalation and associated rate-limiting step are still unclear on the molecular level. Here, by using density functional theory, we evaluate the thermodynamics stability of graphene edge on transition metal surface in the context of oxygen and explore various reaction pathways and energy barriers, from which we can identify the key steps as well as the roles of metal passivated graphene edges during the oxygen intercalation. Our calculations suggest that in well-controlled experimental conditions, oxygen atoms can be easily intercalated through either zigzag or armchair graphene edges on metal surface, whereas the unwanted graphene oxidation etching can be suppressed. Oxygen intercalation is, thus, an efficient and low-damage way to decouple graphene from a metal substrate while it allows reusing metal substrate for graphene growth.

Coating of h-BN onto carbon nanotubes induces polarization at interfaces, and charges become localized at N and C atoms. Field emission of coated tubes is found to be highly stable, and current density fluctuates within 4%. Study further reveals that the electric field established between coatings and tubes facilitates charge transfer across interfaces and electrons are emitted through occupied and unoccupied bands of N and B atoms.

Characterization of protein secondary structures at interfaces is still challenging due to the limitations of surface-selective optical techniques. Here, we address the challenge of characterizing parallel β-sheets by combining chiral sum frequency generation (SFG) spectroscopy and computational modeling. We focus on human islet amyloid polypeptide aggregates and a de novo designed short polypeptide at lipid/water and air/glass interfaces. We find that parallel β-sheets adopt distinct orientations at various interfaces and exhibit characteristic chiroptical responses in the amide I and N-H stretch regions. Theoretical analysis indicates that the characteristic chiroptical responses provide valuable information on the symmetry, orientation, and vibrational couplings of parallel β-sheet at interfaces.

We produced kilometer-long, endlessly parallel, spontaneously piezoelectric and thermally stable poly(vinylidene fluoride) (PVDF) micro- and nanoribbons using iterative size reduction technique based on thermal fiber drawing. Because of high stress and temperature used in thermal drawing process, we obtained spontaneously polar γ phase PVDF micro- and nanoribbons without electrical poling process. On the basis of X-ray diffraction (XRD) analysis, we observed that PVDF micro- and nanoribbons are thermally stable and conserve the polar γ phase even after being exposed to heat treatment above the melting point of PVDF. Phase transition mechanism is investigated and explained using ab initio calculations. We measured an average effective piezoelectric constant as -58.5 pm/V from a single PVDF nanoribbon using a piezo evaluation system along with an atomic force microscope. PVDF nanoribbons are promising structures for constructing devices such as highly efficient energy generators, large area pressure sensors, artificial muscle and skin, due to the unique geometry and extended lengths, high polar phase content, high thermal stability and high piezoelectric coefficient. We demonstrated two proof of principle devices for energy harvesting and sensing applications with a 60 V open circuit peak voltage and 10 μA peak short-circuit current output.

Despite the exceptionally large capacities in Li ion batteries, Si has been considered inappropriate for applications in Na ion batteries. We report an atomic-level study on the applicability of a Si anode in Na ion batteries using ab initio molecular dynamics simulations. While crystalline Si is not suitable for alloying with Na atoms, amorphous Si can accommodate 0.76 Na atoms per Si atom, corresponding to a specific capacity of 725 mA h g(-1). Bader charge analyses reveal that the sodiation of an amorphous Si electrode continues until before the local Na-rich clusters containing neutral Na atoms are formed. The amorphous Na0.76Si phase undergoes a volume expansion of 114% and shows a Na diffusivity of 7 × 10(-10) cm(2) s(-1) at room temperature. Overall, the amorphous Si phase turns out quite attractive in performance compared to other alloy-type anode materials. This work suggests that amorphous Si might be a competitive candidate for Na ion battery anodes.

Heterostructured oxides have shown unusual electrochemical properties including enhanced catalytic activity, ion transport, and stability. In particular, it has been shown recently that the activity of oxygen electrocatalysis on the Ruddlesden-Popper/perovskite (La1-ySry)2CoO4±δ/La1-xSrxCoO3-δ heterostructure is remarkably enhanced relative to the Ruddlesden-Popper and perovskite constituents. Here we report the first atomic-scale structure and composition of (La1-ySry)2CoO4±δ/La1-xSrxCoO3-δ grown on SrTiO3. We observe anomalous strontium segregation from the perovskite to the interface and the Ruddlesden-Popper phase using direct X-ray methods as well as with ab initio calculations. Such Sr segregation occurred during the film growth, and no significant changes were found upon subsequent annealing in O2. Our findings provide insights into the design of highly active catalysts for oxygen electrocatalysis.

The second-order Møller-Plesset perturbation theory (MP2) gradient using resolution of the identity approximation (RI-MP2 gradient) was combined with the fragment molecular orbital (FMO) method to evaluate the gradient including electron correlation for large molecules. In this study, we adopted a direct implementation of the RI-MP2 gradient, in which a characteristic feature of the FMO scheme was utilized. Test calculations with a small peptide presented a computational advantage of the RI-MP2 gradient over the canonical MP2 gradient. In addition, it was shown that the error of the RI-MP2 gradient, caused by RI approximation, was negligible. As an illustrative example, we performed gradient calculations for two biomolecules-a prion protein with GN8 and a human immunodeficiency virus type 1 (HIV1) protease with lopinavir (LPV). These calculations demonstrated that the gradient including the correlation effect could be evaluated with only about twice the computational effort of the Hartree-Fock (HF) gradient.