Computational and Theoretical Techniques for Materials Science (1995) / Chapter Skim
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Chapter 2--Challenges in Materials Research for the Remainder of the Century
Chapter 2--Challenges in Materials Research for the Remainder of the Century
Pages 3-33
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From page 3... ...
Among these acivances are highly efficient methods for local density approximation (LDA) total energy calculations, new pseuclopotentials, a first-principles molecular dynamics (e.g., Car-Parrinello)
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From page 4... ...
calculation of the structure and properties of the fullerenes and fullerides. The Car-Parrinello-type ab initio molecular dynamics approach and the less accurate but less time-consuming tight-binding molecular dynamic schemes have made possible the quantum mechanical calculation of the dynamics and thermodynamical properties of systems in the solid, liquid, and gaseous states.
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From page 5... ...
This is because of the necessarily approximate treatments of many-electron exchangecorrelation effects in these large-scale firstprinciples calculations for real materials. Theoretical clevelopments for better treatment of many-electron effects will thus be another important direction for computational materials research.
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From page 6... ...
OPTICAL PROPERTIES Background Of the many properties of materials, the optical properties are uncloubte(lly among the most useful and interesting, especially for
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From page 7... ...
Linear and nonlinear optical spectroscopies are now common probes for investigating and characterizing the underlying microscopic nature of materials. With devices becoming increasingly smaller and dependent on manmade structures, the relationship between microstructure, defects, and the optical properties of materials is one of increasing importance.
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From page 8... ...
Being basecion ground-state theory, standard electronic structure methods such as the local density functional formalism, which yields excellent results for structural and other ground-state properties of many solids, do not give accurate electron excitation energies. Before the mici-1980s it was not possible to determine from first principles whether a crystal such as Ge was a semiconductor or a metal let alone the quantitative value of its optical band gap anti excitation spectrum.
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From page 9... ...
In the future we can expect a much wider class of nonlinear optical materials to be produced, including, for example, polymers and metallorganic materials. Nanostructures also are a source of nonlinear optical activity.
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From page 10... ...
The advent of ultrahigh vacuum technology and the accelerating theoretical clevelopments in electronic structure calculations, coupled with the emergence of high-performance computing environments, have greatly enhanced our understanding of chemical bonding at solid surfaces and interfaces. Below is a discussion of critical issues germane to the theoretical description of surfaces and interfaces.
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From page 11... ...
the size and complexity of the system 11 that can be described will increase with computer power and memory. Work in this area has aIreacly begun for example, the ab initio density functional calculation of surface reconstruction in Si (!
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From page 12... ...
The most useful theoretical approaches are computational in nature ant! involve ab initio calculations of interatomic potentials, molecular dynamics to probe the short time dynamics, and larger scale kinetic Monte Cario (KMC)
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From page 13... ...
Furthermore, to improve the predictive powers of KMC ant! the ability of this method to provide quantitative results, much work needs to be done to improve our ability to extract these potentials from ab initio calculations or from techniques such as effective medium theory.
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From page 14... ...
However, in other situations, steps may aggregate on the surface or individual steps may become nonuniform, leading to defect trapping, grain boundaries, and inhomogeneous growth. A better understanding of how to manipulate the desirable properties of step growth and prevent the unclesirable properties is of fundamental technological importance.
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From page 15... ...
Either a significantly enhanced density functional theory or elimination of the core electrons using frozen orbitals or pseudopotentials will be neeclec! to clear with that problem.
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From page 16... ...
The computational tools should include molecular dynamics anti Monte CarIo computer simulation programs for atomic, molecular, and polymer systems, as well as visualization software so useful in allowing scientists to "see" the complex processes being simulated on the computer. Specific applications should be chosen so as to hasten tools development and to demonstrate the power and versatility of this computational approach to nanoengineering.
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From page 17... ...
for different arrangements of the nuclei, and the resulting electronic energies, as a function of nuclear position, clefine a potential energy surface that governs the nuclear motion. Armed with a potential energy surface, it is possible in principle to calculate the rates of clynamical processes, such as the rates of chemical reactions or the rates of conformational changes.
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From page 18... ...
For small systems it is also possible to solve the scattering equations quantum mechanically, but this becomes very demanding computationally even for systems with three atoms. A potential energy surface can also be used in following the time evolution of a system by molecular dynamics (MD)
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The correlation effects that arise when bonds are formed or broken are multiconfigurational in nature, and it seems unlikely that LDA can describe these quantitatively. A different coupling of chemical reactions to bulk material is the propagation of shock waves, generates!
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The calculation of such quantities can thus play a role in the design or improvement of nonlinear optical materials. Hyperpolarizabilities are strongly influenced by electron correlation and require very elaborate electronic structure calculations.
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From page 21... ...
Both are consistent with the classical theory of detonations by Zel'dovich, von Neumann, and Doering (ZND:namely, that following shock compression the explosive molecules react and the product molecules expand, represented! by a pressure profile whose principal features are an almost instantaneous shock rise; a von Neumann spike, where reactants are heated and compressed; a reaction zone, where reactions occur accompanied by decreasing density and pressure; and a Taylor wave of the expanding product gases.
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From page 22... ...
In contrast to laboratory experiments, molecular dynamics simulations are carried out at the right time ant! distance scales to resolve these rapid events a sharp shock rise, leading to a von Neumann spike, followed by a Taylor wave expansion.
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From page 23... ...
Ifthe reaction zone and therefore the failure diameter increase as expected, it may become necessary to use scalable parallel computing resources. As more complex molecules are used in these simulations, there will be even greater challenges and opportunities in the area of designing realistic potentials, as well as the need for increased computer power.
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From page 24... ...
Recent advances in computer hardware and algorithms to extend the size and time scales of computations in each of these areas make it possible to imagine integrating them. For example, massively parallel molecular dynamics calculations are now feasible for tens of millions of atoms, thus extending three-dimensional calculations from length scales of nanometers to tenths of micrometers.
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From page 25... ...
Whereas the properties and responses of atomic building blocks are limited, those of the mesoscopic entities are far more diverse and adaptable. Exploiting this wider range of properties enables a very favorable approach to developing materials with given desirable properties, which is to first design the properties of these entities and then assemble them into macroscopic materials.
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in use today. The new lightweight performance-enhancing structural materials for aircraft, and also automobiles, are composite materials.
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that, with the current developments in large N (~1,000 atom) electronic structure capabilities in density functional theory, direct calculations will be available for single building block entities, an entity with surrounding environment, anti even the simpler composite systems.
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In this manner it will be possible to study realistic specific examples of interfaces. The examples for stucly will be established by using molecular dynamics or grant!
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From page 29... ...
Attempts to incorporate local atomic relaxations, charge transfer, or clustering have just begun and remain to be fully tested. The CPA is well suited to the quantum mechanical description of the totalenergy variation associated with compositional variations of chemical species on a common crystalline lattice, order-disorder, and chemical mixing energies, for example.
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From page 30... ...
f electrons, such as rare-earth compounds. It is also unfortunate that, unlike the theory of the optical properties, there is no practical extension of the LSD approximation analogous to the GW theory of excitations.
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From page 31... ...
STRONGLY INTERACTING SYSTEMS Background Electron correlation effects play a critical role in certain classes of materials, such as magnets and superconductors. The ah initio treatment of electron correlation effects in real materials remains one of the most challenging tasks, both conceptually and numerically.
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Another direction has been to broaden the original Hubbard model. The modifications inclu(ie (1)
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From page 33... ...
In acIdition to the lattice mociels described above, Monte Carlo methods (either variational or fixed-nocle cliffusion) have been successfully 33 applied to the treatment of real materials with long-range Coulomb interaction, such as the covalent semiconductor and metals.
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