Geometry Relaxation Research Articles

Geometry, absorption and luminescence of small Ar clusters

We investigate the lowest excitonic levels of small argon clusters, , by means of the Hole-Particle Pseudopotential (HPP) formalism introduced by Dupláa and Spiegelmann. This formalism allows us to model the excited states associated to Rydberg orbitals with higher accuracy than previous studies based on the Diatomics-In-Molecule (DIM) approach, as used by Naumkin and Wales. In contrast to the DIM method, the HPP formalism predicts the excitation to localise mostly on two atoms rather than three, for the relaxed geometry of the lowest triplet state. The hole localisation on a dimer is associated to the appearance of a large electric dipole, which depends on the isomer geometry. This dipole indicates the strong Pauli repulsion experienced by the Rydberg electron, which pushes it out of the cluster. For clusters at their ground state equilibrium geometry, we observe the formation of a singlet-state band associated with the Ar orbital, well separated from the upper states, which matches quite well with the so-called surface exciton band observed experimentally.

A theoretical investigation of uranyl covalencyviasymmetry-preserving excited state structures

Time dependent density functional theory (TDDFT) calculations have been performed on a series of symmetry-preserving excited states of the uranyl dication, UO22+. The simulated excited state electronic structures are compared to that of the ground state at both ground and excited state-optimised geometries. For the first time, the Quantum Theory of Atoms in Molecules (QTAIM) has been applied to the excited states electronic structures of uranyl in order to quantify the variation in bond covalency upon electronic excitation. QTAIM analysis of vertical excitations at the ground state geometry demonstrated an inverse relationship between the orbital mixing coefficient, λ, and the excitation energy. Furthermore, it was found that, for MOs with U 5f character, λ was more dependent on the metal-ligand Hamiltonian matrix element HML, whereas for those with U 6d character, λ became increasingly dependent on the difference in fragment orbital energy levels, ΔEML. Charge transfer from O to U reduced as the excitation energy increased, as did the degree of electron sharing between the centres. When considering the relaxed excited state geometries, a relationship between excitation energy and bond elongation was established, commensurate with the large magnitude of λ and its dependence on HML for MOs with U 5f character, and enhanced charge transfer otherwise.

Arbitrarily accurate quantum alchemy.

Doping compounds can be considered a perturbation to the nuclear charges in a molecular Hamiltonian. Expansions of this perturbation in a Taylor series, i.e., quantum alchemy, have been used in the literature to assess millions of derivative compounds at once rather than enumerating them in costly quantum chemistry calculations. So far, it was unclear whether this series even converges for small molecules, whether it can be used for geometry relaxation, and how strong this perturbation may be to still obtain convergent numbers. This work provides numerical evidence that this expansion converges and recovers the self-consistent energy of Hartree-Fock calculations. The convergence radius of this expansion is quantified for dimer examples and systematically evaluated for different basis sets, allowing for estimates of the chemical space that can be covered by perturbing one reference calculation alone. Besides electronic energy, convergence is shown for density matrix elements, molecular orbital energies, and density profiles, even for large changes in electronic structure, e.g., transforming He3 into H6. Subsequently, mixed alchemical and spatial derivatives are used to relax H2 from the electronic structure of He alone, highlighting a path to spatially relaxed quantum alchemy. Finally, the underlying code that allows for arbitrarily accurate evaluation of restricted Hartree-Fock energies and arbitrary order derivatives is made available to support future method development.

Optical Activity of Metal Nanoclusters Deposited on Regular and Doped Oxide Supports from First-Principles Simulations.

We report a computational study and analysis of the optical absorption processes of Ag20 and Au20 clusters deposited on the magnesium oxide (100) facet, both regular and including point defects. Ag20 and Au20 are taken as models of metal nanoparticles and their plasmonic response, MgO as a model of a simple oxide support. We consider oxide defects both on the oxygen anion framework (i.e., a neutral oxygen vacancy) and in the magnesium cation framework (i.e., replacing Mg++ with a transition metal: Cu++ or Co++). We relax the clusters’ geometries via Density-Functional Theory (DFT) and calculate the photo-absorption spectra via Time-Dependent DFT (TDDFT) simulations on the relaxed geometries. We find that the substrate/cluster interaction induces a broadening and a red-shift of the excited states of the clusters, phenomena that are enhanced by the presence of an oxygen vacancy and its localized excitations. The presence of a transition-metal dopant does not qualitatively affect the spectral profile. However, when it lies next to an oxygen vacancy for Ag20, it can strongly enhance the component of the cluster excitations perpendicular to the surface, thus favoring charge injection.

Theoretical Study of Oxygen Atom Adsorption on A Polycyclic Aromatic Hydrocarbon Using Density-Functional Theory

Potential energy curves (PECs) and energy profiles of atomic O attack on coronene as a model for graphene/graphitic surface and interstellar reaction surface have been computed at the unrestricted B3LYP/cc-pVDZ level of theory to elaborate on atomic O attack mechanism and chemisorption on coronene. The PECs were generated by scanning the O atom distance to the closest carbon atom on "top" and "bridge" positions in the coronene, while fully relaxed geometries in the triplet state were investigated to gain the energy profile. We found that the most favorable geometry as the final product was the chemically bound O on the "bridge" site in the singlet state with an interaction energy of –29.2 kcal/mol. We recommended a plausible mechanism of atomic O attack and chemisorption reaction on coronene or generally graphitic surface starting from the non-interacting O atom and coronene systems into the chemically bound O atom on coronene.

Polarization-sensitive tunable multi-band terahertz absorber based on single-layered graphene rings

In this paper, a metamaterial absorber is proposed, which is constructed by graphene rings and a gold film separated by an ultrathin S i O 2 layer. The feature of this absorber is that the absorption bands can be adjusted either by applying external electric fields or by rotating the polarization angles of the incident electromagnetic waves. The calculation results show that the continuous tunable or two- to multi-band absorptions can be realized by the above two methods. Through equivalent medium theory and impedance-matching condition, the anisotropic absorption mechanism of this absorber is explained. Moreover, the simple specific design makes the absorption bands able to be further tuned by adjusting various parameters, such as the geometry sizes and relaxation times of graphene rings. Our results indicate that the absorber has great adjustability and great potential application in filtering, terahertz signal detection, signal parameter estimation, smart sensing, tunable absorbers, and cloaking.

X-ray transient absorption reveals the 1Au (n\u03c0*) state of pyrazine in electronic relaxation

Electronic relaxation in organic chromophores often proceeds via states not directly accessible by photoexcitation. We report on the photoinduced dynamics of pyrazine that involves such states, excited by a 267 nm laser and probed with X-ray transient absorption spectroscopy in a table-top setup. In addition to the previously characterized 1B2u (ππ*) (S2) and 1B3u (nπ*) (S1) states, the participation of the optically dark 1Au (nπ*) state is assigned by a combination of experimental X-ray core-to-valence spectroscopy, electronic structure calculations, nonadiabatic dynamics simulations, and X-ray spectral computations. Despite 1Au (nπ*) and 1B3u (nπ*) states having similar energies at relaxed geometry, their X-ray absorption spectra differ largely in transition energy and oscillator strength. The 1Au (nπ*) state is populated in 200 ± 50 femtoseconds after electronic excitation and plays a key role in the relaxation of pyrazine to the ground state.

Geodesic structure and quasinormal modes of a tidally perturbed spacetime

Tidal perturbations play an important role in the study of the dynamics in the classical two-body system. Understanding tidal effects in strong-field regions may allow one to use gravitational-wave or electromagnetic observations to locate or constraint the location of possible companions. Here, we investigate how timelike and null geodesics of a Schwarzschild black hole are affected in the presence of a companion. There is a panoply of new effects. In some limiting cases, we find analytical solutions for closed null or timelike geodesics. Our results show that light ring period as measured by a far-away observer can be either shorter or longer, depending on the location of the companion. We also show that there are closed lightlike trajectories which are elliptic (for equatorial companions), and that timelike particles are affected in a similar manner. Finally, we attempt at estimating the ringdown from tidally perturbed geometries. Our results indicate that there are two stages in the relaxation of such geometries, one associated with a prompt decay of waves around the deformed photonsphere, and a later relaxation of the global geometry. These results are consistent with previous, full numerical studies.

EMM-25: The Structure of Two-Dimensional 11 × 10 Medium-Pore Borosilicate Zeolite Unraveled Using 3D Electron Diffraction

The structure of the novel medium-pore borosilicate zeolite EMM-25 has been determined by continuous rotation electron diffraction (cRED). EMM-25 crystallizes in the space group Cmcm with unit cell parameters a = 11.055, b = 22.912, and c = 24.914 Å and a composition of |C4H8(C11H25N)2|2[Si112.5B3.5O232]. The EMM-25 framework possesses a two-dimensional channel system composed of 10-ring channels connected via 11-ring windows. Its channel system is analogous to that of the medium-pore zeolite NU-87 framework but with 11- rather than 12-ring windows, suggesting a different shape selectivity. EMM-25 was first obtained using 1,4-bis(N-methyl-N,N-dihexylammonium)butane as an organic structure directing agent (OSDA). Based on a molecular docking study of the OSDA within the pores of the determined framework structure, a new ammonium dication OSDA with an improved fit was devised. By using this new OSDA, the synthesis time was reduced 80%, from 52 to just 10 days. Furthermore, cRED data revealed a structural disorder of the EMM-25 framework present as swinging zigzag chains. The introduction of the disorder, which is a consequence of geometry relaxation, was crucial for an accurate structure refinement. Lastly, the cRED data from as-made EMM-25 showed residual potential consistent with the location of the OSDA position determined from the Rietveld refinement, concluding a complete refinement of the as-made structure based on the cRED data.

Ab initio molecular dynamics study of AlCl4− adsorption on PEDOT conducting polymer chains

In the search for alternatives to lithium batteries, aluminium makes a promising negative electrode due to its high theoretical specific energy and energy density. One battery chemistry making use of an aluminium negative electrode is the aluminium–poly(3,4-ethylenedioxythiophene) (PEDOT) battery, which has been shown to have long cycle life and specific energy comparable to other aluminium rechargeable batteries. The battery stores AlCl4− anions in the PEDOT cathode when charged. However, the storage mechanism is not well understood. Here, ab initio molecular dynamics simulations (AIMD) are used to help understand the optimum (relaxed) configuration of AlCl4− anions when stored on a single chain of PEDOT. Two main conclusions arise. Firstly, it is generally not stable to have two anions adsorbed to one monomer unit, and this configuration can be avoided for future work. Secondly, AIMD does not find lower energy configurations for the PEDOT/AlCl4− system than DFT geometry relaxation, providing that the starting geometry does not have two anions on the same monomer unit. Based on our results, we believe it is likely that similar behaviour will be observed in other conducting polymer systems.

Anion photoelectron spectroscopic and relativistic coupled-cluster studies of uranyl dichloride anion, UO[formula omitted]Cl[formula omitted

A joint relativistic coupled-cluster and experimental photoelectron (PE) spectroscopic study of the uranyl dichloride anion, UO2Cl2−, is reported. Sophisticated electronic-structure calculations predict the photodetachment of UO2Cl2− to involve a U 5f electron and to be followed by significant geometry relaxation. Therefore, the adiabatic electron affinity (EAa) of the uranyl dichloride neutral molecule, UO2Cl2, and the vertical detachment energy (VDE) of its anion, UO2Cl2−, provide valuable information about its uranium 5f orbital energies. The EAa value was computed to be 3.15 eV. The VDE value was calculated to be 3.55 eV by augmenting the computed EAa with a shift derived from a Franck–Condon simulation using coupled-cluster potential energy surfaces. The VDE, which corresponds to the highest intensity peak in the PE spectrum, was measured to be 3.69 ± 0.20 eV, in good agreement with the computed value. The origin transition in the PE spectrum, whose electron binding energy corresponds to the EAa, was assigned to the feature at 3.2 ± 0.20 eV, consistent with the computed EAa.

Electron-transport and gas sensing in armchair graphene nanoribbons by density functional method

Findings regarding the application of gas sensing on graphene based material have led to jump in nanotechnology. Hence, producing the high-efficiency materials in this technology is a priority. Recent development in graphene based material shows that it can be a good candidate for this tar-get. With this in mind, we theoretically studied the effect of different gas molecule absorption on armchair graphene nanoribbon. In order to identify, the electronic transport of armchair graphene nanoribbons AGNR (8) by using non-equilibrium Green’s function technique, the TRANSIESTA package was used. In order to study the geometry relaxation of device, Spanish Initiative for Electronic Simulations with Thousands of Atoms (SIESTA) package was applied. Also, the generalized gradient approximation of Perdew, Burke and Ernzerhof (PBE) were chosen for the exchange correlation density function. For studying the gas sensing properties of AGNR numerical computations for CO, NO, CO2, and NH3gas molecules were adsorbed on the AGNR (8). Besides, the average local density of states (LDOS) and the transmission spectra T (E, V) for gas molecules adsorbed on the AGNR (8) were plotted. The current was measured as a function of the voltage of applied bias and the transmission spectra was investigated to see the effects of all configurations in the presence of gas molecules. Our results are encouraging and leading experimental research to develop high-quality graphene materials for well-organized applications base on gas sensing.

Crystal Structure of BaCa(CO3)2 Alstonite Carbonate and Its Phase Stability upon Compression.

New single-crystal X-ray diffraction experiments and density functional theory (DFT) calculations reveal that the crystal chemistry of the CaO–BaO–CO2 system is more complex than previously thought. We characterized the BaCa(CO3)2 alstonite structure at ambient conditions, which differs from the recently reported crystal structure of this mineral in the stacking of the carbonate groups. This structural change entails the existence of different cation coordination environments. The structural behavior of alstonite at high pressures was studied using synchrotron powder X-ray diffraction data and ab initio calculations up to 19 and 50 GPa, respectively. According to the experiments, above 9 GPa, the alstonite structure distorts into a monoclinic C2 phase derived from the initial trigonal structure. This is consistent with the appearance of imaginary frequencies and geometry relaxation in DFT calculations. Moreover, calculations predict a second phase transition at 24 GPa, which would cause the increase in the coordination number of Ba atoms from 10 to 11 and 12. We determined the equation of state of alstonite (V0 = 1608(2) Å3, B0 = 60(3) GPa, B′0 = 4.4(8) from experimental data) and analyzed the evolution of the polyhedral units under compression. The crystal chemistry of alstonite was compared to that of other carbonates and the relative stability of all known BaCa(CO3)2 polymorphs was investigated.

First principle study of electronic properties of ZnO nanoclusters with native point defects during gas adsorption

First principle investigations of adsorption processes of different gas molecules (NH3, CO, O2, NO2) on native point defects in (ZnO)n nanoclusters (n = 34, 60) based on the density functional theory within the general gradient approximation plus Hubbard U corrections (GGA + U), for different types of isomers, were performed. For each type of defect an optimization (relaxation) of structures geometry was performed and then the molecules of O2, CO, NO2, NH3, had been added to the surface. The values for adsorption energy, band gap energy, as well as, the charge transfer, and the bond length between the adsorbed molecule and the surface atom for each cluster were obtained to establish the influence of the defects on the electronic properties of the (ZnO)n nanoclusters during gas adsorption. The calculations showed the change in the concentration of majority carriers (electrons) in the sensory system depending on the type of gas adsorbed: CO and NH3 molecules increase the concentration, while O2 and NO2 molecules decrease their concentration. A reduction of the band gap of nanoclusters was also observed during adsorption of molecules. The sharpest decrease for acceptor molecules was observed for the NO2 molecule, and among donor molecules the greatest influence was exerted by NH3 molecules.

Flavin mimetics: Synthesis and photophysical properties

We report the synthesis of new isoalloxazines using a microwave-assisted approach to make N -substituted-2-nitroanilines followed by one-pot reduction and condensation via Hemmerich’s method. The influence of substituents on positions 7, 8, and 10 of flavin core on the optical properties is investigated. The aliphatic functionalities on N10 give rise to quantum yields of 0.7, while aromatic side-chains quench fluorescence. Relaxed geometries (DFT) of chiral and achiral derivatives have been used for TD-DFT calculations, which yielded good agreement with the experimental UV and CD data.

Tuning the Coordination Geometry and Magnetic Relaxation of Co(II) Single-Ion Magnets by Varying the Ligand Substitutions

Three mononuclear Co(II) complexes with the formulas of [Co(L1)2] (1), [Co(L2)2(CH3OH)2] (2), and [Co(L3)2(CH3OH)2] (3) (HL1 = 4-nitro-2-((E)- (propylimino)methyl)phenol, HL2 = 2,4-dinitro-6-((E)-(...

Fragmentation pathways of methylbenzoate cations following core excitation: Theoretical approach using graph theory

We performed theoretical calculations to elucidate the detailed pathways following the core excitation of methylbenzoate. The classical molecular dynamics were expanded to include geometry relaxation on the resonant core-excited state at the O K-edge. The fragmentation pathways of cationic state as the Auger final state were examined. We found that the chemical bonds around methoxy oxygen are dissociative on the core-excited state; furthermore, the yields of fragment ions produced by bond scission of methoxy oxygen increased. Thus, OCH3+ and CH3+ ions were dissociated selectively after core excitation.

Point defects in tight binding models for insulators

We consider atomistic geometry relaxation in the context of linear tight binding models for point defects. A limiting model as Fermi-temperature is sent to zero is formulated, and an exponential rate of convergence for the nuclei configuration is established. We also formulate the thermodynamic limit model at zero Fermi-temperature, extending the results of [H. Chen, J. Lu and C. Ortner, Thermodynamic limit of crystal defects with finite temperature tight binding, Arch. Ration. Mech. Anal. 230 (2018) 701–733]. We discuss the non-trivial relationship between taking zero temperature and thermodynamic limits in the finite Fermi-temperature models.

Electronic and Magnetic Structures of New Interstitial Boron Sub-Oxides B12O2:X (X = B, C, N, O).

The boron-rich boron sub-oxide rhombohedral B6O considered in B12O2 full formulation has a large O-O spacing of ~3 Å and a central vacant position that can receive interstitial atoms X, forming a central O-X-O alignment in the dodecaboron cage as observed in well-known triatomic B12 compounds as B12{C-C-C}, B12{N-B-N}, etc. Plane wave density functional theory (DFT) based calculations of unrestricted geometry relaxation of B12{O-X-O}, X = B, C, N, and O let one identify new ternary sub-oxides, all found cohesive while showing different d(X-O) distances ranging from d(B-O) = 1.95 Å down to d(O-O) = 1.73 Å with intermediate d(C-O) = 1.88 Å. The different magnitudes were assigned to the chemical affinities of X-inserts versus host oxygen with the increasing development of X-O bonding along the series with larger cohesive B12{O-O-O}. From the atom projected charge density, B presents none, while significant magnitudes are shown on C and N, the latter developing bonding with terminal oxygen atoms especially N. The presence of unpaired valence electrons leaves nonbonding charge density on X = C, N interstitial compounds, which, besides the relative isolation of the central C and N lead to the onset of magnetic moments: M(C) = 1.9 μB, and M(N) = 1 μB in a ferromagnetic ground state. Atom-resolved assessments are provided with the magnetic charge density and electron localization function electron localization function (ELF) projections on one hand and the site and spin projected density of states and the chemical bonding based on the overlap integral Sij within the COOP criterion, on the other hand.

Correlated Triplet Pair Formation Activated by Geometry Relaxation in Directly Linked Tetracene Dimer (5,5'-Bitetracene).

Singlet fission (SF) materials have the potential to overcome the traditional external quantum efficiency limits of organic light-emitting diodes (OLEDs). In this study, we theoretically designed an intramolecular SF molecule, 5,5′-bitetracene (55BT), in which two tetracene units were directly connected through a C–C bond. Using quantum chemical calculation and the Fermi golden rule, we show that 55BT undergoes efficient SF induced by geometry relaxation in a locally excited singlet state, 1(S0S1). Compared with another high-performing SF system, the tetracene dimer in the crystalline state, 55BT has advantages when used in doped systems owing to covalent bonding of the two tetracene units. This feature makes 55BT a promising candidate triplet sensitizer for near-infrared OLEDs.