One of the outstanding advancements in electronic-structure density-functional methods is the Sankey-Niklewski (SN) approach [Sankey and Niklewski, Phys. Rev. B 40, 3979 (1989)]; a method for computing total energies and forces, within an ab initio tight-binding formalism. Over the past two decades, several improvements to the method have been proposed and utilized to calculate materials ranging from biomolecules to semiconductors. In particular, the improved method (called FIREBALL) uses separable pseudopotentials and goes beyond the minimal sp 3 basis set of the SN method, allowing for double numerical (DN) basis sets with the addition of polarization orbitals and d-orbitals to the basis set. Herein, we report a review of the method, some improved theoretical developments, and some recent application to a variety of systems.ß 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 Introduction With the increase in computational power, greater efforts have been made by the electronicstructure community to optimize the performance of quantum mechanical methods. Quantum mechanical methods have become increasingly reliable as a complementary tool to experimental research. A variety of methods exist ranging in complexity from semi-empirical methods to density-functional theory (DFT) methods to highly-accurate methods going beyond the one-electron picture. Judicious approximations enable the computational materials science community to more efficiently examine a wider range of materials questions.Otto F. Sankey was one of the early visionaries by, firstly, demonstrating that molecular-dynamics (MD) simulations can be coupled efficiently with electronic-structure methods to optimize structures and evaluate energetics of materials [1]. Secondly, his judicious approximations in the

The magnetization reversal in ordered arrays of Co nanowires with tailored hcp-phase texture, controlled by pH synthesis and nanowires length, has been investigated. The angular dependence of coercivity has been experimentally determined for different crystal textures, and the corresponding magnetization reversal mode is interpreted by analytical modelling. The results show that reversal takes place by propagation of a transverse-like domain wall mode. The fitting of experimental and calculated data allows us the quantitative evaluation of the magnetocrystalline anisotropy constant strength whose magnetization easy direction evolves from parallel to the wires toward in-plane orientation with the change of hcp-phase texture. The simple geometry and high aspect ratios of arrays of magnetic nanowires make them a model system for the study of magnetic phenomena in uniaxial nanomagnets for modern devices applications.1,2 While ultrasoft magnetic nanowires (i.e., permalloy) have been exhaustively investigated, Co nanowires form a particularly interesting system as it is a hard magnetic material which magnetic properties strongly depend on their crystal structure (i.e., phases, texture, and grain size). The control of the orientation of hcp-c axis (i.e., magnetization easy axis of magnetocristalline anisotropy, K mc ) results in the control of the Co nanowires effective anisotropy. A strong longitudinal magnetic anisotropy is achieved when the c axis is oriented parallel to the nanowires, so reinforcing the shape anisotropy. Since the magnetization reversal process is determined by the strength and orientation of the effective magnetic anisotropy, its detailed control and understanding will benefit advances in those applications.In Co nanowires prepared by electrochemical route, crystalline structure can be tuned by adjusting fabrication parameters as current density, plating time, pH, pore diameter, or annealing and deposition under external magnetic fields.3-7 Particularly, it has been shown that pH-controlled electroplating enables the switching between fcc and hcp-Co phases, which modifies the magnetization easy axis from parallel to perpendicular to the wires. 6,7 This is typically qualitatively concluded from the differences in the hysteresis loops shape between parallel and perpendicular applied magnetic field configurations.To achieve full understanding of the magnetization reversal defined by a given anisotropy, the study of coercivity and, specifically of its angular dependence, is an useful tool. Different reversal modes can be induced by suitable modification of the K mc parameter. This feature is relevant for the design of hybrid systems, as multilayers of different Co crystallographic structures, and consequently different controlled reversal modes, which is of interest in spintronic and microwaves devices. Previous works on Co/Cu multilayer nanowires 8 show that competing anisotropies can be present in nanowires. The control of magnetic anisotropy in these nanostructured systems is very important both fo...

Cobalt nanowires, 40 nm in diameter and several micrometers long, have been grown by controlled electrodeposition into ordered anodic alumina templates. The hcp crystal symmetry is tuned by a suitable choice of the electrolyte pH (between 3.5 and 6.0) during growth. Systematic high resolution transmission electron microscopy imaging and analysis of the electron diffraction patterns reveals a dependence of crystal orientation from electrolyte pH. The tailored modification of the crystalline signature results in the reorientation of the magnetocrystalline anisotropy and increasing experimental coercivity and squareness with decreasing polar angle of the 'c' growth axis. Micromagnetic modeling of the demagnetization process and its angular dependence is in agreement with the experiment and allows us to establish the change in the character of the magnetization reversal: from quasi-curling to vortex domain wall propagation modes when the crystal 'c' axis tilts more than 75° in respect to the nanowire axis.

The preparation of magnetic nanopillars from anodic alumina templates represents a cheap way to obtain extensive ordered arrays, and thus is very appealing for nanotechnology applications. In this paper we report the preparation of arrays of Co nanopillars with 120 nm height and varying diameter. The high anisotropy of Co offers an additional possibility to control their magnetic properties. The magnetic properties of arrays of Co nanopillars are studied both experimentally and by micromagnetic simulations. Experiment and modeling show crucial changes of hysteresis loops when the diameter is increased. Magnetic data are interpreted considering the change of crystalline structure as well as the influence of geometry. The micromagnetic simulations explain the measured magnetic properties by the role of magnetocrystalline anisotropy and the combined influence of the shape anisotropy and the interactions. They also show the change in the reversal mode with the increased diameter from vortex propagation to curling when the field is applied parallel to the nanopillar axis, and from coherent rotation to curling when it is applied perpendicular.

A double proton transfer reaction in a Guanine-Cytosine (GC) base pair has been proposed as a possible mechanism for rare tautomer (G*C*) formation and thus a source of spontaneous mutations. We analyze this system with free energy calculations based on extensive Quantum Mechanics / Molecular Mechanics simulations to properly consider the influence of the DNA biomolecular environment. We find that, although the G*C* rare tautomer is metastable in the gas phase, it is completely unstable in the conditions found in cells. Thus, our calculations show that a double proton reaction cannot be the source of spontaneous point mutations. We have also analyzed the intrabase H transfer reactions in Guanine. Our results show that the DNA environment 1