Modified embedded-atom interatomic potential for Fe-Ni,Cr-Ni and Fe-Cr-Ni systems

A semi-empirical interatomic potential formalism, the second-nearest-neighbor modified embedded-atom method (2NN MEAM), has been applied to obtaining interatomic potentials for the Fe-Ni, Cr-Ni and Fe-Cr-Ni systems using previously developed MEAM potentials of Fe and Ni and a newly revised potential of Cr. The potential parameters were determined by fitting the experimental data on the enthalpy of formation or mixing, lattice parameter and elastic constant. The present potentials generally reproduced the fundamental physical properties of the Fe-Ni and Cr-Ni alloys. The enthalpy of formation or mixing of the disordered phase at finite temperature and the enthalpy of mixing of the liquid phase are reasonable in agreements with experiment data and CALPHAD calculations. The potentials can be combined with already-developed MEAM potentials to describe Fe-Cr-Ni-based multicomponent alloys. Moreover, the average diffusivities in the unary, some binary and ternary alloys were simulated based on present potential. Good agreement is obtained in comparison with experimental data.     

A modified embedded-atom method interatomic potential for indium

A semi-empirical interatomic potential for indium has been developed based on the MEAM (modified embedded-atom method) formalism. The potential describes various fundamental physical properties (cohesive energy, lattice parameters, elastic constants, structural energy differences, surface energy and relaxation, vacancy formation and diffusion energy, etc.) of indium in good agreement with relevant experimental data and/or first-principles calculations. The potential also describes bulk properties of non-equilibrium structures (fcc and bcc) of indium in good agreement with first-principles calculations. Because the potential formalism is exactly the same as other previously developed MEAM potentials for a wide range of elements, it can be easily extended to multi-component systems such as In–N, In–As, Ga–In and Ga–In–N.     

Construction of modified embedded atom method potentials for the study of the bulk phase behaviour in binary Pt–Rh, Pt–Pd, Pd–Rh and ternary Pt–Pd–Rh alloys

A first attempt is made to simulate the solid part of the phase diagram of the ternary Pt–Pd–Rh system. To this end, Monte Carlo (MC) simulations are combined with the Modified Embedded Atom Method (MEAM) and optimised parameters entirely based on Density Functional Theory (DFT) data. This MEAM potential is first validated by calculating the heat of mixing or the demixing phase boundary for the binary subsystems Pt–Rh, Pt–Pd and Pd–Rh. For the disordered alloy systems Pt–Rh and Pt–Pd, the MC/MEAM simulation results show a slightly exothermic heat of mixing, thereby contradicting any demixing behaviour, in agreement with other theoretical results. For the Pd–Rh system the experimentally observed demixing region is very well reproduced by the MC/MEAM simulations. The extrapolation of the MEAM potentials to ternary systems is next validated by comparing DFT calculations for the energy of formation of ordered Pt–Pd–Rh compounds with the corresponding MEAM energies. Finally, the validated potential is used for the calculation of the ternary phase diagram at 600 K.     

A modified embedded-atom method interatomic potential for the V-H system

An interatomic potential for the vanadium-hydrogen binary system has been developed based on the second nearest-neighbor modified embedded-atom method (2NN MEAM) potential formalism, in combination with the previously developed potentials for V and H. Also, first-principles calculation has been carried out to provide data on the physical properties of this system, which are necessary for the optimization of the potential parameters. The developed potential reasonably reproduces the fundamental physical properties (thermodynamic, diffusion, elastic and volumetric properties) of V-rich bcc solid solution and some of the vanadium hydride phases. The applicability of this potential to the development of V-based alloys for hydrogen applications is discussed.     

Fitting of interatomic potentials without forces: A parallel particle swarm optimization algorithm

We present a methodology for fitting interatomic potentials to ab initio data, using the particle swarm optimization (PSO) algorithm, needing only a set of positions and energies as input. The prediction error of energies associated with the fitted parameters can be close to 1 meV/atom or lower, for reference energies having a standard deviation of about 0.5 eV/atom. We tested our method by fitting a Sutton–Chen potential for copper from ab initio data, which is able to recover structural and dynamical properties, and obtain a better agreement of the predicted melting point versus the experimental value, as compared to the prediction of the standard Sutton–Chen parameters.     

Second nearest-neighbor modified embedded-atom method interatomic potentials for the Co-M (M = Ti,V) binary systems

Interatomic potentials for the Co–Ti and Co–V binary alloy systems have been developed based on the second nearest-neighbor modified embedded-atom method (2NN MEAM) interatomic potential formalism. Newly developed potentials reproduce various structural and thermodynamic properties of the binary alloys in reasonable agreement with experiments, first-principles calculations, and CALPHAD-type thermodynamic assessments. It is emphasized that these potentials can serve as groundwork for atomistic studies on the design of highly efficient trimetallic noble metal catalysts.     

A modified embedded atom method interatomic potential for carbon

A semi-empirical interatomic potential for carbon has been developed, based on the modified embedded atom method formalism. The potential describes the structural properties of various polytypes of carbon, elastic, defect and surface properties of diamonds as satisfactorily as the well-known Tersoff potential. Combined with the Lennard-Jones potential, it can also reproduce the physical properties of graphite and amorphous carbon reasonably well. The applicability of the present potential to atomistic approaches on carbon nanotubes and fullerenes is also shown. The potential has the same formalism as previously developed MEAM potentials for bcc, fcc and hcp elements, and can be easily extended to describe various metal–carbon alloy systems.     

Modified embedded-atom method interatomic potentials for Mg–X (X=Y,Sn, Ca) binary systems

Interatomic potentials for pure Ca and Mg–X (X=Y, Sn, Ca) binary systems have been developed on the basis of the second nearest-neighbor modified embedded-atom method (2NN MEAM) formalism. The potentials can describe various fundamental physical properties of pure Ca (bulk, defect and thermal properties) and the alloy behavior (structural, thermodynamic and defect properties of solid solutions and compounds) of binary systems in reasonable agreement with experimental data or first-principles and other calculations. The applicability of the developed potentials to atomistic investigations of the deformation behavior of Mg and its alloys is discussed together with some challenging points that need further attention.     

Atomistic Modeling of pure Mg and Mg–Al systems

Interatomic potentials for pure Mg and the Mg–Al binary system have been developed based on the modified embedded-atom method (MEAM) potential formalism. The potentials can describe various fundamental physical properties of pure Mg (bulk, point defect, planar defect and thermal properties) and alloy behaviors (thermodynamic, structural and elastic properties) in reasonable agreement with experimental data or higher-level calculations. The applicability of the potential to atomistic investigations on the deformation behavior of pure Mg and the effect of alloying element Al on it is discussed.     

Second nearest-neighbor modified embedded-atom method interatomic potentials for the Pt-M (M = Al,Co, Cu,Mo, Ni,Ti, V) binary systems

Interatomic potentials for Pt-M (M = Al, Co, Cu, Mo, Ni, Ti, V) binary systems have been developed on the basis of the second nearest-neighbor modified embedded-atom method (2NN MEAM) formalism. The parameters of pure Mo have also been newly developed to solve a problem in the previous 2NN MEAM potential in which the sigma and α-Mn structures become more stable than the bcc structure. The potentials reproduce various materials properties of alloys (structural, thermodynamic and order-disorder transition temperature) in reasonable agreements with relevant experimental data and other calculations. The applicability of the developed potentials to atomistic investigations for the shape and atomic configuration of Pt bimetallic nanoparticles is demonstrated.     

Post Hartree-Fock and density functional theory studies on structure and conformational stability of nitrosoethylene and substituted compounds of nitrosoethylene

Post Hartree-Fock and density functional theory (DFT) methods were used to study the different conformers of nitrosoethylene H-CH=CH-NO, and substituted compounds of the nitrosoethylene R-CH=CH-NO (R = Cl, NH2, N(CH3)2, OH, OCH3). The molecules were optimized at MP2/6-31G* level of theory of ab initio and B3LYP/6-31G* and B3PW91/6-31G* levels of theory of DFT. Special emphasis has been given to the effect of substitution of pi-electron donor groups NH2, N(CH3)2, OH, and OCH3, which play a major role in modifying the geometrical parameters of -N=O group by the electronic transmission effects through the central group -CH=CH-. The ability of DFT methods to predict the bond length adjacent to the atoms having lone pair electrons has been discussed. The conformational stabilities have been studied using the relative energies and DFT parameters such as chemical hardness and chemical potential. The role of intra-molecular hydrogen bond on the equilibrium structure has been discussed. The vibrational spectra for the different conformers of the nitrosoethylene and substituted compounds have been generated using the MP2/6-31G* level of theory.     

Application of genetic algorithm for unknown parameter estimations in cylindrical fin

This article deals with the application of the genetic algorithm (GA) for optimizing an inverse problem and retrieving unknown parameters in cylindrical fin geometry. Parameters such as the thermal conductivity and the heat transfer coefficient are attempted for estimation in order to satisfy a desired temperature field in the medium. The study is done for single-parameter and simultaneous two-parameter retrievals. The temperature field is calculated from a forward problem using the finite difference method using some known values of the properties. These properties are ultimately retrieved by an inverse approach using the GA. The study is done for different controlling parameters such as the number of generations, measurement errors and number of measurement locations. For two parameter simultaneous estimation, many combination of unknown parameters are observed to satisfy a given temperature field, and their ratio is only found to be successfully estimated. The present work is proposed to be useful for selecting the thermal properties which are required to satisfy a given temperature field.     

Molecular potential energies in dodecahedron cell of methane hydrate and dispersion correction for DFT

The interaction potential energies of water-water and water-methane in structure-I unit cell of methane hydrate are calculated from 2.1 to 8.0A using density functional theory (DFT) B3LYP/TZVP. The curves of potential energies are corrected for basis set superposition error (BSSE) and dispersion interaction using a 4-term L-J (4,6-8,12) correction equation, which is derived from CCSD(T)/cc-pVTZ calculations of water-water and water-methane molecular pairs, using least squares curve-fitting. The methane hydrate unit cell is a regular water dodecahedron cell consisting of 20 water molecules with a methane molecule in the center. The geometries of water and methane are optimized at CCSD(T)/cc-pVTZ level. The BSSE-corrections are calculated for water-water and water-methane interaction energies as functions of the side length, l, of the dodecahedron cell at B3LYP/TZVP level in the range from 2.1 to 8.0A. The BSSE CP-corrected and dispersion-corrected potential energy surfaces (PES) of water-water and water-methane are useful for molecular dynamics simulation of gas clathrate-hydrates.     

Modified embedded-atom method interatomic potential for the Mg–Zn–Ca ternary system

Mg–Zn–Ca alloys are representative Mg alloys with high formability at room temperature. Their high formability is thought to be related to slip, twinning, and recrystallization of the alloys, but the detailed mechanisms have not yet been clarified. To enable atomistic simulations for investigating those behaviors, an interatomic potential for the Mg–Zn–Ca ternary system was developed. The development was based on the second nearest-neighbor modified embedded-atom method formalism, combining previously developed Mg–Zn and Mg–Ca potentials with the newly developed Zn–Ca binary potential. The Zn–Ca and Mg–Zn–Ca potentials reproduce structural, elastic, and thermodynamic properties of compounds and solution phases of relevant alloy systems in reasonable agreement with experimental data, first-principles and CALPHAD calculations. The applicability of the developed potentials is demonstrated through calculations of the effects of Zn and Ca solutes on the generalized stacking fault energy for various slip systems, segregation energy on twin boundaries, and volumetric misfit strain.     

Calculation of charges from photoelectron spectra

A computer program to calculate charges on the various atoms in a molecule using experimentally determined binding energies from X-ray photoelectron spectra is described. The method is based on the charge potential model with the incorporation of electronegativities. A program written in BASIC suitable for execution on IBM PC compatible computers is presented, along with two auxiliary programs for the projection of a molecule and for the calculation of interatomic distances within two given moieties.     

Estimating soil thermal properties from sequences of land surface temperature using hybrid Genetic Algorithm–Finite Difference method

Most models used in land surface hydrology, vadose zone hydrology, and hydro-climatology require an accurate representation of soil thermal properties (soil thermal conductivity and volumetric heat capacity). Various empirical relations have been suggested to estimate soil thermal properties. However, they require many input parameters such as soil texture, mineralogical composition, porosity and water content, which are not always available from laboratory experiments and field measurements. In this paper, to overcome the above challenge, a hybrid numerical method, Genetic Algorithm–Finite Difference (GA–FD), is proposed to estimate soil thermal properties using land surface temperature (LST) as the only input. The genetic algorithm (GA) optimization method coupled with the finite difference (FD) modeling technique is a viable hybrid approach for estimating soil thermal properties. The finite difference method is employed to solve the heat diffusion equation and simulate LST, while a robust optimization technique (GA) is used to retrieve soil thermal properties by minimizing the difference between observed and simulated LST. Furthermore, a generalization of the hybrid model is developed for inhomogeneous soil, in which soil thermal properties are not constant throughout the soil slab. The proposed model is applied to the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE). The results show that the proposed hybrid numerical method is able to estimate soil thermal properties accurately, and therefore effectively eliminate the need for the unavailable soil parameters which are required by empirical methods for determining the soil thermal conductivity and volumetric heat capacity. Remarkably, the temporal variation of the retrieved soil thermal conductivity is consistent with the volumetric water content, even though no water content information is used in the model.     

铁卟啉化合物与氧气分子形成的体系的密度泛函理论研究(英文)

采用密度泛函理论B3LYP方法在6-31G(d)基组水平下对氯化铁(+3)卟啉与氧气分子形成的体系进行了研究,得到了几何构型,电子性质及分子轨道结构等相关数据.对两个体系不同自旋状态下的几何构型参数和电子性质对比发现:受体系立体构型对称性的影响,在两个体系中凡是与卟啉环上N原子相关的几何参数及电子性质均呈现出相同规律性.又采用密度泛函理论UB3LYP/6-311G*//UB3LYP/6-31G*方法对这两个体系不同自旋状态下的能量进行了计算,分析表明自旋多重度越高体系越稳定.然后分别分析了两个体系在最稳定自旋状态下的分子轨道占据情况及中心Fe原子最外层3d轨道的电子分布情况,结果表明Fe原子的3d二和3dxz/3dyz与氧气分子的单占据反键轨道HOMO π*2px/π*2py之间存在相互作用,这种相互作用引起铁卟啉环与O2分子间的电子转移并使O2活化.然而,根据分析在通常状态下铁卟啉对O2分子的活化作用是微弱的.