Large Folds Research Articles

Analysis of an antenna system design for a synthetic L- and P-band aperture radar

The article presents an analysis of options for designs of the antenna system for a synthetic aperture space radar (SAR), with acceptable performance characteristics in L- and P-bands. At the first stage the analysis of the current state of producing spacecraft with L- and P-band radars that use quite large folding antenna structures is carried out. It is shown that to obtain high performance of SAR in L- and P-band large folding reflectors with the diameter of up to15 meters are used. Further, variants of antenna systems are discussed and compared to determine the best combination in terms of the characteristics obtained and design complexity. An antenna system is proposed in the paper that uses antennae of the Yagi-aerial type as antenna array elements. The possibility of creating a space system consisting of several unified small spacecraft with L- and P-band SAR on board is shown.

The emplacement of the Peridotite Nappe of New Caledonia and its bearing on the tectonics of obduction

The Peridotite Nappe of New Caledonia is one of the few ophiolites worldwide that escaped collisional orogeny after obduction. Here, we describe the deformation associated with serpentinization in two klippes of the nappe in northwestern New Caledonia. The klippes are flat-lying and involve S/SW-vergent reverse-slip shear zones which are true compressional structures in origin. Further northeast, the nappe is folded in association with the development of a steep schistosity in low grade metasediments. This difference in structural style indicates that the Peridotite Nappe experienced compression at greater depths toward its root zone, suggesting a “push from the rear” mechanism of emplacement. This supports the view that the nappe has been emplaced through horizontal contraction sustained by plate convergence. We establish a crustal-scale cross-section at the end of the obduction event, before Neogene extension. This involves a large fold nappe of high pressure rocks bounded from below by a major thrust. Furthermore we show that obduction in New Caledonia occurred through dextral oblique convergence. Oblique convergence probably resulted from the initial obliquity between the subduction trench and the continental ribbon that became incorporated in it. This obliquity can solve the paradox of the Peridotite Nappe seemingly being emplaced at the same time the high pressure rocks were exhumed. Oblique convergence together with focused erosional denudation on the northeastern flank of the island led to exhumation of the metamorphic rocks in a steep fold nappe rising through the rear part of the orogen.

Failure analysis of a welded steel pipe at Kullar fault crossing

Abstract The seismic response of a 2200-mm-diameter welded steel pipe at strike slip Kullar fault crossing in Izmit, Kocaeli during 1999 Kocaeli earthquake is investigated. The pipe was crossing the fault-line with an angle of 55° and suffered leaks due to 3.0 m of right lateral movement of fault, which imposed compressive axial strain in the pipe. The backfill material of the trench was native soil which was non-homogenous (soft and stiff clay) with respect to fault line-soft material on the North side, stiff material on the South side. Field observations revealed two major wrinkles with finger width cracks and a minor wrinkle on the soft soil side of the fault. Large plastic strains and local folding were observed at wrinkles due to compressive strains. The case is known as one of the best documented fault crossing examples. The failure behavior of the Thames water pipe during 1999 Kocaeli earthquake is simulated by utilizing a 3D nonlinear continuum FE model. The numerical model considers contact surface at soil pipe interface and performs large deformation analyses of the pipe. The locations of wrinkles as well as axial displacements/rotations demands due to fault rupture are predicted. It is observed that once wrinkle initiates, strain in the pipe away from the wrinkle reduces after initial local buckling and additional shortening of the pipeline tends to accumulate at the wrinkle causing large plastic strains and rotation demands associated with fault rupture, an observation consistent with field observations and 2005 ALA guidelines.

Mesozoic tectono-magmatic evolution of Mexico: An overview

The Geological Framework of Mexico is formed by a mosaic of tectono-stratigraphic terranes that were assembled during the Paleozoic and Mesozoic as the result of the complex interaction between Laurentia, Gondwana, and the paleo-Pacific plate. Even though the Mexican tectono-magmatic evolution is still object of study, some main representative stages can be recognized. Determining the time span and regional distribution of magmatism throughout the geological history of Mexico is crucial for the exploration of mineral resources. This paper contains a brief description of the terranes and is focused on summarizing the main Mesozoic tectono-magmatic events, from the assembling of Pangea in Late Paleozoic time to the end of the Mesozoic. These main events are: 1) Permo-Triassic continental (submarine) arc that developed along eastern Oaxaquia and was related to a subduction zone developed along the paleo-pacific margin. 2) Magmatic activity ceased in central and southern Mexico, during Middle to Late Triassic time, and a wide submarine fan was formed along the western margin of Pangea, which apparently acted like a passive margin. Evidence of a contemporaneous intra-oceanic arc is found at Baja California. 3) Inland Early Jurassic volcanism is found in northeast and central Mexico. Whether this magmatic event evolved in a rift or arc (Nazas) tectonic setting is still undetermined. 4) By Late Jurassic, magmatism was widespread. It is the time of the breakup of Pangea, leading to the opening of the Gulf of Mexico and the Arperos Basin. Therefore, volcanism was mostly related to subduction and supra-subduction rifting. 5) The Gulf of Mexico and Arperos basins were formed by Early Cretaceous time. Igneous rocks of that age have a wide range of composition (arc, continental rift, strike slip, and oceanic rift, within plate), and are geographically widespread. 6) At the end of Early Cretaceous, volcanism moved to a more localized belt along the paleo Pacific margin; it is also the time of formation of major carbonate platforms along east Mexico. 7) Coeval to Sevier-Laramide orogeny of North America, a large fold and thrust belt developed throughout Mexico, during Late Cretaceous time. It was accompanied by arc magmatism along the pacific margin of Mexico (Tarahumara Arc). It is the first continental arc that evolved after accretion and consolidation of Guerrero and Alisitos terranes to Continental Mexico. These Mesozoic tectonic processes originated westward-eastward migrations of arc-related and rift-related volcanism, which in turn lead to the present distribution of Mesozoic mineral deposits.

The electronic and transport properties of the folded zigzag graphene nanoribbon

The geometrical structures, electronic and transport properties of the folded zigzag graphene nanoribbon were investigated by using nonequilibrium Green's functions and the density-functional theory. It was found that the electronic and transport properties are determined by the folding degree of the zigzag graphene nanoribbon. The I-V curve is nearly linear for the large folding degree. The negative differential conductance in the zigzag graphene nanoribbon disappears with the large folding degree, and this is valuable for the rational design of the graphene-based electronic devices.

Derivation of the area balance relations of detachment folds from first principles

The shortening and depth to detachment are two fundamental parameters for the construction, balancing, and restoration of geological cross-sections across fold and thrust systems. The principle of mass conservation requires both parameters to scale as the excess (uplifted) area above the original depositional level. In many occasions, however, the excess area appears to be less than the displaced (contracted) area, a phenomenon that is thought to be the product of bulk area loss, and layer thickening.We present a derivation of Chamberlin's relation of detachment folds in which the excess area equals shortening multiplied by the depth to detachment by means of a local area-balance approach. We work out corrections for changes in bed thickness and layer-parallel compaction. Our corrections indicate the missing linear shortening scales as the wavelength of the fold. Thus, larger folds should display larger strain deficits than smaller ones. Additionally, we demonstrate that the missing strain scales as the transported area as well given rocks are compressible materials. Consequently, Chamberlin's classical relationship only holds for low-strain, relatively small detachment folds. Under this condition layer-parallel compaction, and changes in bed thickness are negligible.We further analyze and work out corrections for the case in which sedimentary layers gradually narrow with horizontal distance; we find the stratal geometry has a strong effect on the area balance of detachment folds. Our results are in agreement with previous works, and suggests a more accurate approximation to estimate the shortening is by means of a best fit model in which the response variable is the excess area and the regressor is the average layer thickness.

Structure of Arabidopsis thaliana FUT1 Reveals a Variant of the GT-B Class Fold and Provides Insight into Xyloglucan Fucosylation.

The plant cell wall is a complex and dynamic network made mostly of cellulose, hemicelluloses, and pectins. Xyloglucan, the major hemicellulosic component in Arabidopsis thaliana, is biosynthesized in the Golgi apparatus by a series of glycan synthases and glycosyltransferases before export to the wall. A better understanding of the xyloglucan biosynthetic machinery will give clues toward engineering plants with improved wall properties or designing novel xyloglucan-based biomaterials. The xyloglucan-specific α2-fucosyltransferase FUT1 catalyzes the transfer of fucose from GDP-fucose to terminal galactosyl residues on xyloglucan side chains. Here, we present crystal structures of Arabidopsis FUT1 in its apoform and in a ternary complex with GDP and a xylo-oligosaccharide acceptor (named XLLG). Although FUT1 is clearly a member of the large GT-B fold family, like other fucosyltransferases of known structures, it contains a variant of the GT-B fold. In particular, it includes an extra C-terminal region that is part of the acceptor binding site. Our crystal structures support previous findings that FUT1 behaves as a functional dimer. Mutational studies and structure comparison with other fucosyltransferases suggest that FUT1 uses a SN2-like reaction mechanism similar to that of protein-O-fucosyltransferase 2. Thus, our results provide new insights into the mechanism of xyloglucan fucosylation in the Golgi.

Bearing Length on Cold-Formed Sections

The effective bearing length of trapezoidal sheeting on cold formed sections at inner supports is 10 mm according to EN 1999-1-4 (aluminium) and EN 1993-1-3 (steel). In the original design provisions the effective bearing length on Z-sections was the actual width of the loaded flange. In order to find out the appropriate effective bearing length, FEM calculations were made on simply supported beams with C-, Z-and Sigma-cross-section. Contact elements between the trough of the trapezoidal sheeting and the loaded flange of the beam made it possible to evaluate the contact area. This area and the stresses in the trapezoidal sheeting show that the effective bearing length is very small for C-sections. For Z-sections and for Sigma sections with large folds in the web the contact area is the flange width, unless the flange width versus profile height is large and the plate thickness is small.

Large Scale Chromosome Folding Is Stable against Local Changes in Chromatin Structure.

Characterizing the link between small-scale chromatin structure and large-scale chromosome folding during interphase is a prerequisite for understanding transcription. Yet, this link remains poorly investigated. Here, we introduce a simple biophysical model where interphase chromosomes are described in terms of the folding of chromatin sequences composed of alternating blocks of fibers with different thicknesses and flexibilities, and we use it to study the influence of sequence disorder on chromosome behaviors in space and time. By employing extensive computer simulations, we thus demonstrate that chromosomes undergo noticeable conformational changes only on length-scales smaller than 105 basepairs and time-scales shorter than a few seconds, and we suggest there might exist effective upper bounds to the detection of chromosome reorganization in eukaryotes. We prove the relevance of our framework by modeling recent experimental FISH data on murine chromosomes.

Enhancing antibody folding and secretion by tailoring the Saccharomyces cerevisiae endoplasmic reticulum.

BackgroundThe yeast Saccharomyces cerevisiae provides intriguing possibilities for synthetic biology and bioprocess applications, but its use is still constrained by cellular characteristics that limit the product yields. Considering the production of advanced biopharmaceuticals, a major hindrance lies in the yeast endoplasmic reticulum (ER), as it is not equipped for efficient and large scale folding of complex proteins, such as human antibodies.ResultsFollowing the example of professional secretory cells, we show that inducing an ER expansion in yeast by deleting the lipid-regulator gene OPI1 can improve the secretion capacity of full-length antibodies up to fourfold. Based on wild-type and ER-enlarged yeast strains, we conducted a screening of a folding factor overexpression library to identify proteins and their expression levels that enhance the secretion of antibodies. Out of six genes tested, addition of the peptidyl-prolyl isomerase CPR5 provided the most beneficial effect on specific product yield while PDI1, ERO1, KAR2, LHS1 and SIL1 had a mild or even negative effect to antibody secretion efficiency. Combining genes for ER enhancement did not induce any significant additional effect compared to addition of just one element. By combining the Δopi1 strain, with the enlarged ER, with CPR5 overexpression, we were able to boost the specific antibody product yield by a factor of 10 relative to the non-engineered strain.ConclusionsEngineering protein folding in vivo is a major task for biopharmaceuticals production in yeast and needs to be optimized at several levels. By rational strain design and high-throughput screening applications we were able to increase the specific secreted antibody yields of S. cerevisiae up to 10-fold, providing a promising strain for further process optimization and platform development for antibody production.Electronic supplementary materialThe online version of this article (doi:10.1186/s12934-016-0488-5) contains supplementary material, which is available to authorized users.

Superficial Mycoses Associated with Diaper Dermatitis.

Diapers create particular conditions of moisture and friction, and with urine and feces come increased pH and irritating enzymes (lipases and proteases). Fungi can take advantage of all these factors. Candida yeasts, especially C. albicans, are responsible for the most frequent secondary infections and are isolated in more than 80 % of cases. Correct diagnosis is important for ensuring the correct prescription of topical antimycotics. Nystatin, imidazoles and ciclopirox are effective. It is important to realize there are resistant strains. Dermatophytes can infect the diaper area, with the most common agent being Epidermophyton floccosum. The clinical characteristics of dermatophytosis are different from those of candidiasis, and it can be diagnosed and treated simply. Malassezia yeasts can aggravate conditions affecting the diaper area, such as seborrheic dermatitis, atopic dermatitis, and inverse psoriasis. Additional treatment is recommended in this case, because they usually involve complement activation and increased specific IgE levels. Erythrasma is a pseudomycosis that is indistinguishable from candidiasis and may also occur in large skin folds. It is treated with topical antibacterial products and some antimycotics.

Design Optimization Challenges of Origami-Based Mechanisms With Sequenced Folding

Reconfigurable structures based on origami design are useful for multifunctional applications, such as deployable shelters, solar array packaging, and tunable antennas. Origami provides a framework to decompose a complex 2D to 3D transformation into a series of folding operations about predetermined foldlines. Recent optimization toolsets have begun to enable a systematic search of the design space to optimize not only geometry but also mechanical performance criteria as well. However, selecting optimal fold patterns for large folding operations is challenging as geometric nonlinearity influences fold choice throughout the evolution. The present work investigates strategies for design optimization to incorporate the current and future configurations of the structure in the performance evaluation. An optimization method, combined with finite-element analysis, is used to distribute mechanical properties within an initially flat structure to determine optimal crease patterns to achieve desired motions. Out-of-plane and twist displacement objectives are used in three examples. The influence of load increment and geometric nonlinearity on the choice of crease patterns is studied, and appropriate optimization strategies are discussed.

Overturned folds in ice sheets: Insights from a kinematic model of traveling sticky patches and comparisons with observations

AbstractOverturned folds are observed in regions of the Greenland ice sheet where driving stress is highly variable. Three mechanisms have been proposed to explain these folds: freezing subglacial water, traveling basal slippery patches, and englacial rheological contrasts. Here we explore how traveling basal sticky patches can produce overturned folds. Transitions from low to high stress cause a tradeoff in ice flow between basal slip and internal deformation that deflects ice stratigraphy vertically. If these transitions move, the slip‐deformation tradeoff can produce large folds. Those folds record the integrated effects of time‐varying basal slip. To understand how dynamic changes in basal slip influence ice sheet stratigraphy, we develop a kinematic model of ice flow in a moving reference frame that follows a single traveling sticky patch. The ice flow field forms a vortex when viewed in the moving reference frame, and this vortex traps ice above the traveling patch and produces overturned folds. Sticky patches that travel downstream faster produce larger overturned folds. We use the model as an interpretive tool to infer properties of basal slip from three example folds. Our model suggests that the sticky patches underneath these folds propagated downstream at rates between one half and the full ice velocity. The regional flow regime for the smaller two folds requires substantial internal deformation whereas the regime for the largest fold requires substantially more basal slip. The distribution and character of stratigraphic folds reflect the evolution and propagation of individual sticky patches and their effects on ice sheet flow.

Changes of myoid and endothelial cells in the peritubular wall during contraction of the seminiferous tubule.

The wall of the seminiferous tubule in rodents consists of an inner layer of myoid cells covered by an outer layer of endothelial cells. Myoid cells are a type of smooth muscle cell containing α-actin filaments arranged in two independent layers that contract when stimulated by endothelin-1. The irregular surface relief of the tubular wall is often considered a hallmark of contraction induced by a variety of stimuli. We examine morphological changes of the rat seminiferous tubule wall during contraction by a combination of light, confocal, transmission and scanning electron microscopy. During ET-1-induced contraction, myoid cells changed from a flat to a conical shape, but their actin filaments remained in independent layers. As a consequence of myoid cell contraction, the basement membrane became wavy, orientation of collagen fibers in the extracellular matrix was altered and the endothelial cell layer became folded. To observe the basement of the myoid cell cone, the endothelial cell monolayer was removed by collagenase digestion prior to SEM study. In contracted tubules, it is possible to distinguish cell relief: myoid cells have large folds on the external surface oriented parallel to the tubular axis, whereas endothelial cells have numerous cytoplasmic projections facing the interstitium. The myoid cell cytoskeleton is unusual in that the actin filaments are arranged in two orthogonal layers, which adopt differing shapes during contraction with myoid cells becoming cone-shaped. This arrangement impacts on other components of the seminiferous tubule wall and affects the propulsion of the tubular contents to the rete testis.

Variation in Female Reproductive Tract Morphology of the Common Bottlenose Dolphin (Tursiops truncatus).

Cetaceans exhibit vaginal folds, unusual protrusions of the vaginal wall into the vaginal lumen. Inconsistent terminology and a lack of anatomical landmarks in the literature have hindered comparative studies of the form and function of vaginal folds. Our objectives are to: (1) develop a standardized measurement protocol for the reproductive tracts of female cetaceans, (2) assess variation in morphometrics within the common bottlenose dolphin (Tursiops truncatus), and (3) determine if vaginal muscle is skeletal, and therefore of somatic origin in this species. We selected 15 measurements to characterize female reproductive tracts and evaluated variability using fresh or frozen-thawed specimens from southeastern USA representing a range of sexual maturity states and reproductive states (n = 18 specimens). Presence of skeletal muscle and variation in the density of muscle banding were assessed using 90 histological samples (n = 5 specimens). Analyses of the gross morphological data revealed that the dolphins generally had one large vaginal fold that bisected the vaginal lumen. Vaginal morphology was similar for sexually mature and immature specimens and across reproductive states. The histological data revealed that the vaginal musculature consisted of smooth muscle, consistent with other mammals, leading us to conclude that vaginal contractions are likely under autonomic rather than somatic control. No differences were found in the density of smooth muscle banding among vaginal regions or sexual maturity states. Our systematic protocol lays the foundation for evaluating the function (e.g., sexual selection, natural selection) and evolution of vaginal folds.

A novel application of electrospinning technique in sublingual membrane: characterization, permeation and in vivo study

Isosorbide dinitrate–polyvinylpyrrolidone (ISDN–PVP) electrospinning fibers were formulated and explored as potentially sublingual membrane. The addition of polyethylene glycol (PEG) to the formulation improved flexibility and reduced fluffiness of the fiber mat. The scanning electron microscopy (SEM) demonstrated that the fibers tended to be cross-linking, and the crosslinking degree increased with the increase of PEG amount. The differential scanning calorimetry (DSC) indicated that ISDN existed in non-crystalline state in the fibers (except at the highest drug content). The infrared spectroscopy suggested that ISDN had better compatibility with the ingredients owing to the hydrogen bonding (or hydrophobic interactions). The fibers were highly favorable for the fabrication of sublingual membrane due to neutral pH, large folding endurance and rapid drug release (complete dissolution within 120 s). The permeation study of ISDN through both dialysis membrane (DM) and porcine sublingual mucosa (SM) were carried out. A significant relationship of drug permeation rate through DM and SM was built up, which indicated that DM could be used to partly simulate SM and assess formulation. The pharmacokinetic study in rats demonstrated that the electrospinning fiber membrane had a higher Cmax and lower Tmax compared to the reference preparation, and the relative bioavailability of the fiber membrane was 151.6%.

Morphometric analysis of young petiole galls on the narrow-leaf cottonwood, Populus angustifolia, by the sugarbeet root aphid, Pemphigus betae

An insect-induced gall is a highly specialized structure resulting from atypical development of plant tissue induced by a reaction to the presence and activity of an insect. The insect induces a differentiation of tissues with features and functions of an ectopic organ, providing nutrition and protection to the galling insect from natural enemies and environmental stresses. In this anatomical and cytological study, we characterized how the gall-inducing aphid Pemphigus betae reshapes the leaf morphology of the narrow-leaf cottonwood Populus angustifolia to form a leaf fold gall. Young galls displayed a bend on one side of the midvein toward the center of the leaf and back to create a fold on the abaxial side of the leaf. This fold was formed abaxially by periclinal and anticlinal divisions, effectively eliminating intercellular spaces from the spongy parenchyma. Galls at this stage exhibited both cell hypertrophy and tissue hyperplasia. Cells on the adaxial surface were more numerous and smaller than cells near the abaxial surface were, creating the large fold that surrounds the insect. Mesophyll cells exhibited some features typical of nutritive cells induced by other galling insects, including conspicuous nucleolus, reduced and fragmented vacuole, smaller and degraded chloroplasts, and dense cytoplasm compared to ungalled tissue. Even though aphids feed on the contents of phloem and do not directly consume the gall tissue, they induce changes in the plant vascular system, which lead to nutrient accumulation to support the growing aphid numbers in mature galls.

Advanced Dynamic & Crash Simulation of Lightweight Profiles for Design of Roadside Infrastructure

Many research works recently have attempted to use different computational and numerical simulation techniques to model the material thermal large deformation processes for the design of high performance profiles in new roadside infrastructure designs. The material processes for the lightweight crash-capable parts are among the most delicate processes for the material scientist and designing engineers. The forming and extrusion of lightweight alloys involves thermal effects, large deformation, complex geometries and free surface boundaries. The conventional approach towards the simulation of extrusion process using Finite Element (FE) or Finite Volume (FV) has serious short comes even when updated Lagrangian, Eulerian or ALE techniques are employed. During past decades, there has been considerable effort to simulate the whole extrusion process by splitting it into steady state (using Eulerian technique) and transient (using Updated Lagrangian technique) processes.The damage initiation, progression and also failure of lightweight hollow profiles during crash are a result of accumulated damage under plastic deformation. Based on the damage theories, as the loading condition is changing for the material, a plastic deformation may take place which would progressively increase the damage in the component. The accumulated damage would ultimately result in the failure of the cross-section. There are different numerical models to calculate the damage evolution, fracture initiation and also its propagation using continuum and/or discrete damage techniques. In the present study, following an in-depth study of material processing and its absorption capacities, folding modes and geometric/production constrains; a frame work has been setup to develop and test an optimised aluminium extruded profiles for best dynamic and crash performance characteristics. The numerical dynamic simulations (including fatigue, vehicle buffeting …) and virtual crash performance of lightweight hollow profiles have been considered for the design of new generation of roadside signals, lighting posts… for future.The complicated mathematical basis of large deformations, plasticity, contact and folding have been developed and special attention has been devoted to the plastic deformation, rate dependency and tailored yield locus. To assess the dynamic performance and energy absorption of these profiles, a full transient dynamic analysis can be performed using a time history dynamic loading. The new absorbing component design has been checked and verified using a result of carefully-setup experiments work and also advanced explicit simulation runs. One of the main contributions of this paper is to show the applicability and reliability of the numerical simulation approach for the crash performance of new lightweight roadside entities.

Multi-Temporal InSAR Processing Comparison in Presence of High Topography

In this paper we compare the results obtained by three different InSAR tool packages, StaMPS, SARPROZ and P-SBAS (CNR-IREA) on G-POD for the processing of a mountainous area with up to 2,000 m high differences. For that we use two dataset ERS-1/2 and Envisat ASAR. The area (Sierra Tejeda, southern Spain) is located close to the coast line in the Internal Zone of the Betic Cordillera. The recent NW-SE convergence of the Eurasian-African plate boundary makes this area to belong to a large fold which continues active up to present. The uplift of the mountain ranges is mainly related to the development of folds in this regional compressive setting. This paper investigates and compares the velocity pattern obtained by the three different algorithms to help to constraint the geological hazard in this part of the Betic Cordillera.

Large Ligand Folding Distortion in an Oxomolybdenum Donor–Acceptor Complex

Interligand charge transfer is examined in the novel metallo-dithiolene complex MoO(SPh)2((i)Pr2Dt(0)) (where (i)Pr2Dt(0) = N,N'-isopropyl-piperazine-2,3-dithione). The title complex displays a remarkable 70° "envelope"-type fold of the five-membered dithiolene ring, which is bent upward toward the terminal oxo ligand. A combination of electronic absorption and resonance Raman spectroscopies have been used to probe the basic electronic structure responsible for the large fold-angle distortion. The intense charge transfer transition observed at ∼18 000 cm(-1) is assigned as a thiolate → dithione ligand-to-ligand charge transfer (LL'CT) transition that also possesses Mo(IV) → dithione charge transfer character. Strong orbital mixing between occupied and virtual orbitals with Mo(x(2)-y(2)) orbital character is derived from a strong pseudo Jahn-Teller effect, which drives the large fold-angle distortion to yield a double-well potential in the electronic ground state.