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Molecular Electronics Devices
Published in Sergey Edward Lyshevski, Nano and Molecular Electronics Handbook, 2018
Ab initio methods are often called first principles calculations. The distinguishing feature from the other, semi-empirical, type is that the setup and algorithms do not use any fitting parameters, but formally start from the geometry of the system and universal constants like electron mass, charge, etc. Although the quality of the results depends on the degree of approximations used, the obtained result is free from any empirical estimations and expectations. The advantage is that the approximations used in the numerical methods and theoretical model can—at least in principle (and often in practice)—to a very large degree be improved on, and thus the convergence of the result checked. This provides a rigorous assessment procedure for the estimation of the quality of the results. The price paid in practice for the ab initio background of the theory is the complexity of the calculations that consequently demand significant computational resources.
Computational Modeling of Nanoparticles
Published in Sarhan M. Musa, ®, 2018
The problem associated with the SW potential outlined above is that most empirical potentials are fitted to a limited set of properties and usually give a poor description of the properties. Moreover, empirical potentials do not provide any electronic structure information. A possible way out of this dilemma is offered by the ab initio potentials. These potentials provide the most accurate way to calculate atomic forces, electronic properties, and vibrational properties. The majority of these methods make use of the LDA within the limits of the DFT. However, substantial computational effort is required, thus limiting the applicability of ab initio methods to relatively small systems, in practice up to 100 atoms.
Bohmian Pathways into Chemistry: A Brief Overview
Published in Xavier Oriols, Jordi Mompart, Applied Bohmian Mechanics, 2019
Electronic structure methods (e.g., valence bond theory, ab initio methods, density functional theory, etc.) are aimed at obtaining properties associated with the electronic configuration, such as the chemical bonding and intermolecular forces [81]. Basically all these methods are based on quantum mechanics and are included under what is known as quantum chemistry. Molecular dynamics and statistical mechanics methods are based either in classical mechanics or in quantum mechanics, and essentially constitute the scope of the chemical physics and physical chemistry. Molecular dynamics methods, e.g., wave packet propagation methods, are used to describe the properties associated with the nuclear motion (dynamics), necessary to understand chemical reaction processes, for example. In this case, the role of electrons manifests through effective potential functions, namely potential energy surfaces, generated from their bonding to the nuclei. At this point, it is worth highlighting that, for practical purposes, a division has been considered between a few degree-of-freedom treatments, which are henceforth denoted as molecular dynamics methods, and those to deal with many-body problems, which will be referred to as statistical mechanics methods. This convenient classification allows us to distinguish between methods and problems where the full system dynamics is relevant, from those where we are more interested in statistical properties or we only need to focus our attention on a part of the total system (neglecting the remaining “environmental” degrees of freedom). Bearing this in mind, when the number of nuclei involved is relatively large, we shall talk of statistical methods (e.g., molecular dynamics, Monte Carlo, path integrals, etc.), which include methodologies and theories developed to tackle open quantum systems, complex systems, systems far from equilibrium, etc., all of them approximate.
Modelling of short-range ordering kinetics in dilute multicomponent substitutional solid solutions
Published in Philosophical Magazine, 2020
J. Svoboda, D. Holec, M. Popov, G.A. Zickler, F.D. Fischer
The motivation of this paper is to present a general atomistic-statistical-thermodynamic model for the treatment of kinetics of numbers of Cs and Ps in a dilute multicomponent system under varying temperature in a compact way. In contrast to previous statistical mechanics models, the calculation of Gibbs energy of the system is based on the division of the system into subsystems and application of the established Bragg–Williams approximation to these individual subsystems. The state of the system is then described by concentrations of atoms in Cs and Ps considered as independent internal state variables. The bonding energies are calculated by ab initio methods in a standard way. To determine the kinetics of the system, additionally, only tracer diffusion coefficients of all solute components are needed. Therefore, the model combines ab initio calculations with non-equilibrium thermodynamics and can be utilised for an improved determination of the thermodynamic properties of solid solutions accounting also for their thermal history. It is also necessary to be noted that the present kinetic model allows calculations within well acceptable computation times, which cannot be achieved using other kinetic/atomistic methods. To check the accuracy of the present model the results of simulations are compared with simulations by kinetic Monte Carlo Method with a very good agreement.
A thermochemical study on the primary oxidation of sulfur
Published in Combustion Science and Technology, 2019
N. Sebbar, J.W. Bozzelli, H. Bockhorn, D. Trimis
Geometries and standard enthalpies of formation of some reactants, products and transition state structures describing the primary reactions for the combustion of sulfur are reported. In order to find accurate computational methods, DFT methods at several levels and composite ab-initio methods are used. Standard enthalpies are calculated using different isodesmic reactions (hypothetical reactions with similarity in mass and bonding environments for reactant and product sets in order to provide cancellation of systematic errors). Transition state structure (TS) enthalpies were obtained from the computational energy of the TS structure relative to both the reactants and products. Zero-point energies (ZPVEs) and thermal corrections to 298.15 K are applied for all calculations. Frequencies and moments of inertia are obtained from each method in order to calculate the entropies and heat capacities. The latter results are not reported in this study.