Self-consistent finite-difference electronic structure calculations for large systems

An efficient and flexible self-consistent method of solving the Schrödinger equation for large systems is presented. This uses a finite-difference method, with the atomic cores replaced by an embedding potential. The resulting Hamiltonian matrix is sparse and can be diagonalised using the Lanczos algorithm, with computer time proportional to the system size. This all-electron method uses a small muffin tin radius and allows for the full potential outside the muffin tin. Within the self-consistent local density functional framework, Poisson's equation is solved using the multigrid method. Results for fcc copper are shown.     

Linear scaling methods for electronic structure calculations and quantum molecular dynamics simulations

In the past five years, notable advances in the field of electronic structure calculations have been made, by the development of linear scaling methods for total energy calculations and quantum molecular dynamics simulations. These are methods implying a computational workload which grows linearly with th system-size,in contrast to standard algorithms where the workload scales as the cube of the system-size. Therefore the use of linear scaling methods can considerably widen the class of systems and type of problems being tackled with quantum simulations. At present, linear scaling methods using semi-empirical Hamiltonians allow one to perform simulations involving up to a thousand atoms on small workstations, and up to ten thousand atoms for tens of picoseconds when using supercomputers. This has made it possible to study problems such as large organic molecules in water, thin film growth on a surface and extended defects in semiconductors. Although the implementation of first-principles linear scaling methods is less advanced than that of semi-empirical methods, promising results regarding organic molecules and metal-alloys have already appeared in the literature.     

Coherent molecular resonances in quantum dot-metallic nanoparticle systems: coherent self-renormalization and structural effects

It is known that surface-plasmon resonances of metallic nanoparticles can significantly enhance the field experienced by semiconductor quantum dots. In this paper we show that, when quantum dots are in the vicinity of metallic nanoparticles and interact with coherent light sources (laser fields), coherent exciton-plasmon coupling (quantum coherence effects) can increase the amount of the plasmonic field enhancement significantly. We also study how the coherent molecular resonances generated by such a coupling process are influenced by the self-renormalization of the plasmonic fields and the structural parameters of the systems, particularly the size and shape of the metallic nanoparticle. The renormalization process happens via mutual impacts of the radiative decay rate of excitons and the coherent exciton-plasmon coupling on each other. Our results highlight the conditions where the molecular resonances become very sharp, offering optical switching processes with high extinction ratio and wide ranging device applications.     

Efficient light-scattering calculations for aggregates of large spheres

Calculation of the scattering pattern from aggregates of spheres through the T-matrix approach yields high-precision results but at a high-computational cost, especially when the aggregate concerned is large or is composed of large-size spheres. With reference to a specific but representative aggregate, we discuss how and to what extent the computational effort can be reduced but still preserve the qualitative features of the signature of the aggregate concerned.     

Atoms in strong laser fields: challenges in relativistic quantum mechanics

We address the challenges raised by the question of how to produce quantitative data in order to reproduce and to interpret the results of recent and future experiments performed with ultra-intense laser pulses. The main challenges lie in the problem of implementing reliable numerical codes for describing quantum processes experienced by electrons brought in the relativistic regime in the presence of the field.     

Polarization energy calculations in molecular crystals

The self-consistent polarization field (SCPF) and Fourier transform methods of calculating the polarization energy of excess charges are compared. The SCPF method is extended (i) to treat molecules as a set of submolecules rather than single points in the inner region around the charge, and (ii) to treat the outer region around the charge as an anisotropic dielectric continuum rather Ethan an isotropic one. The contribution to the polarization energy from the outer region depends on the average 〈? ^?1〉, where ? is the dielectric tensor. These extensions allow the SCPF method to be used for elongated molecules, with potential applications to various systems lacking translational symmetry.     

Polarization energy calculations in molecular crystals

The self-consistent polarization field (SCPF) and Fourier transform methods of calculating the polarization energy of excess charges are compared. The SCPF method is extended (i) to treat molecules as a set of submolecules rather than single points in the inner region around the charge, and (ii) to treat the outer region around the charge as an anisotropic dielectric continuum rather Ethan an isotropic one. The contribution to the polarization energy from the outer region depends on the average ^–1, where is the dielectric tensor. These extensions allow the SCPF method to be used for elongated molecules, with potential applications to various systems lacking translational symmetry.     

New complementary logic circuits using coupled open quantum systems

This paper present new complementary logic circuits that exploit an intrinsic bistability in nanoscale coupled open quantum systems such as wells and wires. When operated with a multiple-phase split-level clocking scheme, the device can latch a binary digit while meeting gain, tolerance, and I/O decoupling requirements at the same time, which was difficult with conventional back-to-back negative differential resistance devices. This same structure in a dual-rail configuration can function as a complete set of classical logic circuits by changing only the connections of the input signals. When these circuits are combined to form a sequential circuit, low power operation is expected, thanks to better voltage scaling, a zero-standby-power equalized state, and efficient charge recycling.     

First-principles quantum chemistry in the life sciences

The area of computational quantum chemistry, which applies the principles of quantum mechanics to molecular and condensed systems, has developed drastically over the last decades, due to both increased computer power and the efficient implementation of quantum chemical methods in readily available computer programs. Because of this, accurate computational techniques can now be applied to much larger systems than before, bringing the area of biochemistry within the scope of electronic-structure quantum chemical methods. The rapid pace of progress of quantum chemistry makes it a very exciting research field; calculations that are too computationally expensive today may be feasible in a few months' time! This article reviews the current application of 'first-principles' quantum chemistry in biochemical and life sciences research, and discusses its future potential. The current capability of first-principles quantum chemistry is illustrated in a brief examination of computational studies on neurotransmitters, helical peptides, and DNA complexes.     

Crystal density predictions for nitramines based on quantum chemistry

An efficient and convenient method for predicting the crystalline densities of energetic materials was established based on the quantum chemical computations. Density functional theory (DFT) with four different basis sets (6-31G(**), 6-311G(**), 6-31+G(**), and 6-311++G(**)) and various semiempirical molecular orbital (MO) methods have been employed to predict the molecular volumes and densities of a series of energetic nitramines including acyclic, monocyclic, and polycyclic/cage molecules. The relationships between the calculated values and experimental data were discussed in detail, and linear correlations were suggested and compared at different levels. The calculation shows that if the selected basis set is larger, it will expend more CPU (central processing unit) time, larger molecular volume and smaller density will be obtained. And the densities predicted by the semiempirical MO methods are all systematically larger than the experimental data. In comparison with other methods, B3LYP/6-31G(**) is most accurate and economical to predict the solid-state densities of energetic nitramines. This may be instructive to the molecular designing and screening novel HEDMs.     

Error growth in transient large displacement calculations

Nonlinear problems are widely acknowledged as being more difficult to solve numerically than linear problems. Various kinds of errors contribute to this difficulty and in this paper some of these errors will be described and illustrated by solving certain large displacement problems using eight-noded isoparametric brick elements in space and an explicit integration method in time. First, approximation errors in time integration are illustrated, with violation of energy conservation being used as an indicator of the increased difficulties encountered in solving large displacement problems. Next, round-off errors and order of operations are discussed and illustrated for the case of a cube that is impulsively set in rotation about its centre of mass. Simple tests of invariance with respect to translation and rotation are shown to be interesting and potentially useful. Finally, approximation errors in spatial discretization, especially those associated with incomplete or inconsistent integration over the element volume, are illustrated for a large deflection beam problem.     

Two innovations in diffraction calculations for cylindrically symmetrical systems

Two mathematical innovations are presented that relate to calculating propagation of radiation through cylindrically symmetrical systems using Kirchhoff diffraction theory. The first innovation leads to an efficient means of computing Lommel functions of two arguments (u and nu), typically denoted by U(n)(u, nu) and V(n)(u, nu). This can accelerate computations involving Fresnel diffraction by circular apertures or lenses. The second innovation facilitates calculations of Kirchhoff diffraction integrals without recourse to the Fresnel approximation, yet with greatly improved efficiency like that characteristic of the latter approximation.     

Decoherence in quantum systems

We discuss various definitions of decoherence and how it can be measured. We compare and contrast decoherence in quantum systems with an infinite number of eigenstates (such as the free particle and the oscillator) and spin systems. In the former case, we point out the essential difference between assuming "entanglement at all times" and entanglement with the reservoir occuring at some initial time. We also discuss optimum calculational techniques in both arenas.     

Modelling real-time systems: Issues and challenges

In this paper, we discuss the issues and challenges that lie in the specification, development, and verification of real-time systems. In our presentation, we emphasize on the issues underlying modelling of real-time distributed concurrency. Partial support by the Indo-French Centre for the Promotion of Advanced Research/Centre Franco-Indien Pour la Promotion de la Recherche Advancee as part of the project “Formal Specification and Development of Real-Time Reactive Programs” is gratefully acknowledged.     

New challenges in education for sustainable development

Within the context of the Decade of Education for Sustainable Development that began on January 2005, there is growing awareness of the urgency for educators to make changes in the ways they educate and in the content of their courses and curricula. This means that educators must increasingly focus upon multi-disciplinary, multi-generational approaches to help their students, of all ages, to become increasingly skilled and motivated to work within the Seven Generation Mandate (SGM) to help achieve Sustainable Development (SD). To meet the urgent challenges confronting educators, we must address the interconnectedness of ethical, economic and ecological facets of SD. The urgency is underscored in the context of the question, “Do we have the commitment to help our societies make the necessary changes?     

A practical method for accurate quantification of large fault trees

This paper describes a practical method to accurately quantify top event probability and importance measures from incomplete minimal cut sets (MCS) of a large fault tree. The MCS-based fault tree method is extensively used in probabilistic safety assessments. Several sources of uncertainties exist in MCS-based fault tree analysis. The paper is focused on quantification of the following two sources of uncertainties: (1) the truncation neglecting low-probability cut sets and (2) the approximation in quantifying MCSs. The method proposed in this paper is based on a Monte Carlo simulation technique to estimate probability of the discarded MCSs and the sum of disjoint products (SDP) approach complemented by the correction factor approach (CFA). The method provides capability to accurately quantify the two uncertainties and estimate the top event probability and importance measures of large coherent fault trees. The proposed fault tree quantification method has been implemented in the CUTREE code package and is tested on the two example fault trees.     

Using spectral low rank preconditioners for large electromagnetic calculations

For solving large dense complex linear systems that arise in electromagnetic calculations, we perform experiments using a general purpose spectral low rank update preconditioner in the context of the GMRES method preconditioned by an approximate inverse preconditioner. The goal of the spectral preconditioner is to improve the convergence properties by shifting by one the smallest eigenvalues of the original preconditioned system. Numerical experiments on parallel distributed memory computers are presented to illustrate the efficiency of this technique on large and challenging real‐life industrial problems. Copyright © 2004 John Wiley & Sons, Ltd.