Theory and Computational Methods

Presented in part at International Symposium on Atomic, Molecular, and Condensed Matter Theory, and Computational Methods, 37th Sanibel Symposium, (St. Augustine, Florida, USA, 1-7 March, 1997). 

Recent Advances in Nuclear Magnetic Shielding Theory and Computational Methods... 

J. Kohanoff, Electronic Structure Calculations for Solids and Molecules Theory and Computational Methods, Cambridge, New York, 2006. 

Per-Olov Lowdin. Guest Editors N. Yngve Ohrn, John R. Sabin, and Michael C. Zerner, Quantum Chemistry, Solid-State Theory, and Computational Methods. Proceedings of an International Symposium held in Marineland, Florida, March 12-21, 1987, in Int. J. Quantum Chem., Quantum Chem. Symp., No. 21, Wiley, New York, 1987. 

Advances in relativistic quantum theory and computational methods made it possible to predict properties of the heaviest element compounds by performing accurate calculations of their electronic structures. Relativistic atomic and molecular calculations in combination with various models were useful in helping to design sophisticated and expensive chemical experiments. Experimental results, in turn, were helpful in defining the scope of the theoretical problems and provided an important input. The synergism between the theoretical and experimental research in the last decade led to better understanding the chemistry of these exotic species. 

The shear and compressional acoustic wave velocities for the inner core are the direct output parameters from seismological observations. In order to make a direct comparison between the seismic data and measured physical properties, measurements of the acoustic velocities for iron at core pressures are required. Only very recently has it become possible to measure the elastic constants of s-Fe at high pressures and room temperature (Mao etal., 1999 Lubbers etal., 2000 Fiquet et al., 2001 Anderson et at, 2001). Recent advances in theory and computational methods have also provided new tools for computing the elastic constants of s-Fe at core pressures (Stixrude and Cohen, 1995 Soderhnd et al., 1996 Cohen et al., 1997 Steinle-Neumann and Stixrude, 1999) and core conditions (Laio et al., 2000 Steinle-Neumann et al, 2001 Alfe et al., 2001). There is considerable disagreement on the elastic constants of s-Fe between experimental results and theoretical calculations. The dilferences in the aggregate shear (FJ and compressional (Vp) wave velocities are smaller (Hemley and Mao, 2001 Steinle-Neumann et ai, 2001). Further improvement of theory and experiment is required to resolve the discrepancies. 

Copies of the TNO peroxide test databases have been provided to E27.07 and the new versions of CHETAH are expected to contain an extensive database as well as pattern-recognition techniques for estimating the hazard of new materials. The CHETAH software will continue to rely on bond energy data and group contribution calculations to estimate energy release potential. Hopefully, the new versions will also incorporate natural language expert system-type front ends so that the CHETAH program(s) will see expanded use in both analytical and tutorial modes. Copies of the LEILA (8) dissertation have also been provided to E27.07 as an example of an expert system approach to selection and use of appropriate theories and computational methods for the solution of problems in chemical kinetics. 

The distribution of electrons is another element of molecular structure that is very important for understanding chemical reactivity. It is considerably more difficult to obtain experimental data on electron density, but fortunately, in recent years the rapid development of both structural theory and computational methods has allowed such calculations. We make use of computational electron density data in describing molecular structure, properties, and reactivity. In this chapter, we focus on the minimum energy structure of individual molecules. In Chapter 2, we consider other elements of molecular geometry, including dynamic processes involving conformation, that is, the variation of molecular shape as a result of bond rotation. In Chapter 3, we discuss... 

Recent advances in relativistic quantum theory and computational methods have raised the research in the theoretical chemistry of the heaviest elements to a qualitatively new level. It became possible to predict properties of the heaviest elements, their gaseous compounds and complexes in solutions with a sufficiently high accuracy. On the basis of those calculations, the behaviour of the heaviest element species in specific chemical experiments was reliably predicted and confirmed by specially designed experiments. 

The theory and computational methods for the analysis and calculation of field-free resonances in polyelecfronic afoms and molecules that are adumbrated here were initiated in 1972 [11a] as an alternative to the theoretical approaches mentioned above and in the review articles [12-16, 21], for the purpose of dealing efficiently with the MEP in arbitrary electronic structures. A complementary discussion based on the ideas in Ref. [11a] and on later publications dealing with various topics within energy-dependent and time-dependent frameworks was given a few years later [37b]. 

The complete analysis of electron transfer is typically performed in two steps. First the electronic coupling is calculated for a given nuclear configuration of the protein and then the dynamics is included and the averaging and other dynamics effects are incorporated into calculations of electron transfer rates. Various theories that combine pure electronic and dynamic effects are then utilized to calculate the rate of electron transfer [44,52,53]. In recent years, a number of theories and computational methods for electron tunneling in proteins have been developed and reviewed extensively. Of particular interest is the work of the Beratan and Onuchic groups, who pioneered the area of tunneling pathways [54]. 

Expression [8.62] has several merits, but also several defeets. Among the latter we note that given powers of R may collect terms of different origin. For example, the term R" describes both the induction dipole-induced dipole interaetion and the dispersion dipole-dipole interaetion two contributions with a different sensitivity with respeet to ehanges in the molecular system. Theory and computational methods both permit getting two separate coefficients for the two contributions. This is done in a limited number of potentials (the most important cases are the NEMOn and file ASP-Wn families of models). The most popular choice of a unique term for contributions of different origin is motivated by the need of reducing computational efforts, both in the derivation and in the use of the potential. To fit a unique coefficient means to use AE values only for two eoeffieients there is the... 

Kohanoff J (2006) Electrratic stmctuie calculations for solids and molecules theory and computational methods. Cambridge University Press, Cambridge... 

Jameson, C. J. Recent Advances in Nuclear Magnetic Shielding Theory and Computational Methods , in Modeling NMR Chemical Shifts, Gaining Insights into Structure and Environment (ACS Symposium Series 732) Facelli, J. C., Ed., ACS Washington DC, 1999, pp 1-23. 

Part 1, Theory and Computational Methods, opens with a chapter by M. A. Yurkin (Russia) who describes in detail the Discrete Dipole Approximation (DDA) approach, which is an efficient method to study the absorption and scattering of metal nanoparticles of arbitrary shapes. This chapter will serve as an important reference for theoreticians to model metal nanoparticles. Chapter 3 reports DDA results for nanoparticles of different sizes and shapes. This systematic analysis, inspired by recent literature, should represent an important reference for both experimentalists and theoreticians to verify and compare the absorption and scattering spectra of different nanoparticles. While these first two chapters are completely dedicated to metal nanoparticles. Chapter 4 introduces the discussion about the molecular counterpart. In this chapter E. Fabiano (Italy) sheds light on the optical and photophysical... 

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