Modeling Complex Systems by Structural Invariants Approach

When modeling complex systems, we usually encounter the following difficulties: partiality, large amount of data, and uncertainty of conclusions. It can be said that none of the known approaches solves these difficulties perfectly, especially in cases where we expect emergences in the complex system. The most common is the physical approach, sometimes reinforced by statistical procedures. The physical approach to modeling leads to a complicated description of phenomena associated with a relatively simple geometry. If we assume emergences in the complex system, the physical approach is not appropriate at all. In this article, we apply the approach of structural invariants, which has the opposite properties: a simple description of phenomena associated with a more complicated geometry (in our case pregeometry). It does not require as much data and the calculations are simple. The price paid for the apparent simplicity is a qualitative interpretation of the results, which carries a special type of uncertainty. Attention is mainly focused (in this article) on the invariant matroid and bases of matroid (M, BM) in combination with the Ramsey graph theory. In addition, this article introduces a calculus that describes the emergent phenomenon using two quantities-the power of the emergent phenomenon and the complexity of the structure that is associated with this phenomenon. The developed method is used in the paper for modeling and detecting emergent situations in cases of water floods, traffic jams, and phase transition in chemistry

density functional study of the electronic properties of bismuth subcarbonate Bi2O2CO3

Starting from the X-ray diffraction data of Huang et al. we have optimized the atomic positions by minimization of the forces (1 mRy/au) using Perdew-Burke-Ernzerhof generalized gradient approximation (GGA). From the relaxed geometry the electronic structure and the chemical bonding are determined. We have employed the full potential linear augmented plane wave (FPLAPW) method as embodied in the WIEN2k code. In order to get a better estimate of the energy gap we have used the modified Becke-Johnson potential (mBJ) exchange-correlation potential. Our calculations show that the conduction band minimum (CBM) is situated at the centre of the Brillouin zone (BZ) and the valence band maximum (VBM) is located between W and T symmetry points of the BZ, indicating that bismuth subcarbonate is a semiconductor with an indirect band gap of about 0.8 eV. The electrons effective mass ratio (m(e)(*)/me) around G point are calculated. The partial density of states and the electronic charge density distribution shows that there exists a strong covalent bond between C and O atoms. Our calculated bond lengths and angles show very good agreement with the experimental data. (C) 2014 Elsevier Masson SAS. All rights reserve

2D Hexagonal SnTe monolayer: a quasi direct band gap semiconductor with strain sensitive electronic and optical properties

The stability and electronic and optical properties of two-dimensional (2D) SnTe monolayer has been systematically studied by using first-principles calculations based on density functional theory. Our computations demonstrate that the predicted 2D SnTe monolayer is a stable quasi-direct semiconductor. Also, analysis of its electronic property shows that the ground state of this monolayer is a quasi-direct semiconductor with a band gap of ~2.00. This band gap can be effectively modulated by external strains. Investigation of optical properties shows that monolayer SnTe exhibits significant absorption and reflectivity in the ultraviolet region of the electromagnetic spectrum

Thermoelectric properties of Nowotny–Juza NaZnX (X = P, As and Sb) compounds

The electronic transport coefficients of Nowotny-Juza NaZnX (X = P, As and Sb) compounds were evaluated by using the semi-classical Boltzmann and the rigid band model. The compounds are direct band gap (Cv-Cc) semiconductors with energy band gaps of about 1.8, 1.47 and 0.25 eV, for NaZnX (X = P, As and Sb) respectively. A maximum value of electrical conductivity of about 5.4 x 10(20) (Omega ms)(-1) and 5.1 x 10(20) (Omega ms)(-1) is achieved at 0.2 mu (eV) for n-type of NaZnP and NaZnAs, while for NaZnSb a maximum value of about 4.1 x 10(20) (Omega ms)(-1) is found at -0.15 mu (eV) for p-type and 4.2 x 10(20) (Omega ms)(1) at 0.15 mu (eV). The investigated materials exhibit the highest value of Seebeck coefficient at 300 K of about 300, 250, 510 (mu V/K) for p-type and -290, -240, -305 (lV/K) for n-type of NaZnX (X = P, As and Sb) respectively. In the chemical potential region between -/+ 0.6, -/+ 0.5, -/+ 0.025 mu (eV) NaZnP, NaZnAs, NaZnSb respectively exhibit a minimum value of electronic thermal conductivity. Therefore in these regions the investigated materials can give maximum efficiency

Ferroelectric polarization promoted bulk charge separation for highly efficient CO(2) photoreduction of SrBi(4)Ti(4)O(15)

Fast recombination of photogenerated charge carriers in bulk remains the major obstacle for photocatalysis nowadays. Developing ferroelectrics directly as photoactive semiconducting catalysts may be promising in view of the strong ferroelectric polarization that induces the anisotropic charge separation. Here, we report a ferroelectric layered perovskite SrBi₄Ti₄O₁₅ as a robust photocatalyst for efficient CO₂ reduction. In the absence of co-catalysts and sacrificial agents, the annealed SrBi₄Ti₄O₁₅ nanosheets with the strongest ferroelectricity cast a prominent photocatalytic CO₂ reduction activity for CH₄ evolution with a rate of 19.8 μmol h⁻¹ g⁻¹ in the gas-solid reaction system, achieving an apparent quantum yield (AQY) of 1.33% at 365 nm, outperforming most of the reported photocatalysts. The ferroelectric hysteresis loop, piezoresponse force microscopy (PFM) and ns-level time-resolved fluorescence spectra uncover that the outstanding CO₂ photoreduction activity of SrBi₄Ti₄O₁₅ mainly stems from the strong ferroelectric spontaneous polarization along [100] direction, which allows efficient bulk charge separation along opposite direction. DFT calculations also disclose that both electrons and holes show the smallest effective masses along a axis, verifying the high mobility of charge carriers facilitated by ferroelectric polarization. This study suggests that the traditionally semiconducting ferroelectric materials that have long been studied as ferro/piezoelectric ceramics now may be powerfully applied in the photocatalytic field to deal with the growing energy crisis.Shuchen Tu, Yihe Zhang, Ali H. Reshak, Sushil Auluck, Liqun Ye, Xiaopeng Han, Tianyi Ma, Hongwei Huan

Jedno- a dvourozměrné hledání rovnice stavu pomocí nově vydaného balíčku 2DRoptimize

Nový balíček nazvaný 2DRoptimize byl propuštěn pro provádění dvourozměrných vyhledání rovnice stavu (EOS) pro rohové, tetragonální a hexagonální sloučeniny. Balíček je kompatibilní a dostupný s balíčkem WIEN2k. Balík 2DRoptimize zajišťuje optimální optimalizaci objemu a struktury c / a. Za prvé balík najde nejlepší hodnotu pro c / a a přidruženou energii pro každý objem. Ve druhém kroku vypočítá EoS. Balík potom zjistí rovnici poměru c / a vs. objem pro výpočet poměru c / a v optimalizovaném objemu.New package called 2DRoptimize has been released for performing two-dimensional searches of the equation of state (EOS) for rhombohedral, tetragonal, and hexagonal compounds. The package is compatible and available with the WIEN2k package. The 2DRoptimize package performs a convenient volume and c/a structure optimization. First, the package finds the best value for c/a and the associated energy for each volume. In the second step, it calculates the EoS. The package then finds the equation of the c/a ratio vs. volume to calculate the c/a ratio at the optimized volume

First-principles calculations of the elastic, electronic, and optical properties of the filled skutterudites Ce Fe4 P12 and Th Fe4 P12

cited By 38International audienceThe complex density-functional theory calculations of structural, electronic, and optical properties for two principal representatives of the filled skutterudites Ce Fe4 P12 and Th Fe4 P12 have been reported using the full-potential linearized augmented plane-wave method plus local orbitals, as implemented in the WIEN2K code. In this approach, the local-density approximation is used for the exchange-correlation potential. We performed these calculations with and without spin-orbit interactions. Results are given for lattice constant, bulk modulus, and its pressure derivative. Band structure, density of states, pressure coefficients of energy gaps, and refractive indices are also given. We note that both Ce Fe4 P12 and Th Fe4 P12 are semiconductors with indirect and direct energy gaps, respectively. The valence-band maximum is located at Γ for both compounds, whereas the conduction-band minimum is located at Γ for Th Fe4 P12 and at N for Ce Fe4 P12. Our results are compared with previous theoretical calculations and experimental data. © 2007 The American Physical Society