The rich chemistry of boron compounds are often found dominated by its structural dimensionality and chemical bonding from which some of the qualitative features of boron clusters can easily be extracted. In this article, we review such features to discuss structural properties of Bn clusters. In both small-cluster regime of n ≤ 20 and largecluster regime of n ≥ 20, the preferred topological structures are the result of the interplay between bonding factors related to the delocalized π bonds and the inter-icosahedral and intra-icosahedral bonds. The bulk fragments of boron are also expected to become a competitive isomeric configuration with the increase in the cluster-size, in contrast to 3D spherical cages observed in the large carbon clusters
Ab initio calculations were performed to asses the aromatic behavior of mixed tetramer neutral cluster (Be2N2, Be2P2, Mg2N2, Mg2P2). Harmonic vibrational analysis has been performed to ensure the stability of the optimized geometries. The analysis of structure, vibrational frequencies, and molecular orbitals indicates that all these tetramer favor planar atomic configuration as the lowest energy structure and exhibit the characteristics of aromaticity (planarity and two π- electrons in the delocalized molecular orbital). Other than this, the aromatic character of these clusters has been verified based on established criteria of aromaticity such as chemical (extra stability), and magnetic criteria i.e. by calculating Nuclear Independent Chemical Shift (NICS) at the ring centers. The extra electronic stability of these clusters towards donating or accepting of electrons is also reflected in the calculated large ionization potential and low electron affinity
The manner in which the research and development efforts of different groups, each focused on a different aspect of a single complex computing artifact (e.g., database), evolve and mutually support the development of the artifact as a whole has fascinated researchers, economists, and philosophers of science alike. In this paper we propose the Aggregate General Design Cycle (AGDC), an aggregated form of the General Design Cycle (GDC), as a predictive model of the evolution over time of a computing research community of interest. We begin by demonstrating that the GDC accurately depicts the progress of any individual research effort. We then propose that multiple research and development efforts on a theme, even when conducted by nominally distinct groups (i.e. computer science cf. information systems; academics cf. practitioners) are predicted by the AGDC. We provide support for the proposal through a longitudinal meta-bibliographic study of database research.
A fundamental problem in the processing of image sequences is the computation of the velocity field of the apparent motion of brightness patterns usually referred to optical flow. In this paper a novel optical flow estimator based on a bivariate quasi-interpolant operator is presented. Namely, a non linear minimizing technique has been employed to compute the velocity vectors by modeling the flow field with a 2D quasi-interpolant operator based on centered cardinal B-spline functions. In this way an efficient computational scheme for optical flow estimate is provided. In addition the large solving linear systems involved in the process are sparse. Experiments on several image sequences have been carried out in order to investigate the performance of the optical flow estimator.
We discuss some important issues arising when approximating numerically stabilised inverse diffusion processes. We prove rigorously the necessity of a minmod-type stabilisation. Furthermore, we give rigorously verified assertions concerning the occurence of undesirable staircasing aka terracing artefacts. The theoretical results are supplemented by numerical tests.