A first principles simulation of rigid water

We present the results of Car-Parrinello (CP) simulations of water at ambient conditions and under pressure, using a rigid molecule approximation. Throughout our calculations, water molecules were maintained at a fixed intramolecular geometry corresponding to the average structure obtained in fully unconstrained simulations. This allows us to use larger time steps than those adopted in ordinary CP simulations of water, and thus to access longer time scales. In the absence of chemical reactions or dissociation effects, these calculations open the way to ab initio simulations of aqueous solutions that require timescales substantially longer than presently feasible (e.g. simulations of hydrophobic solvation). Our results show that structural properties and diffusion coefficients obtained with a rigid model are in better agreement with experiment than those determined with fully flexible simulations. Possible reasons responsible for this improved agreement are discussed

Outcomes of Deep Brain Stimulation on Adolescent and Adult Women with Refractory Anorexia Nervosa

Background: Anorexia nervosa is a psychosomatic disease that affects many women worldwide. It is commonly associated with comorbidities and has the highest mortality rate of any mental illness. Current treatments are often insufficient in treating this disease and frequently fail to improve the health and quality of life of these women. Deep brain stimulation (DBS) is an established treatment for neurological movement disorders and is emerging as a treatment for psychological disorders similar to anorexia. Due to this similarity, DBS may be an option for treating these women who fail other treatments. This review looks at the outcomes of DBS as a treatment for anorexia nervosa. Methods: An exhaustive search of available medical literature was conducted using Medline-OVID, CINAHL, and Web of Science using the keywords: deep brain stimulation, anorexia nervosa, and female. Relevant articles were assessed for quality using GRADE. Results: Through screening, three studies were found that fit the criteria for this review. This included three observational studies. All three showed improved outcomes in the majority of subjects and demonstrated the relative safety of the procedure. Outcomes included body mass index (BMI), vital signs, anxiety, depression, obsessive-compulsion, quality of life, intelligence, memory, social functioning, brain glucose metabolism, and adverse events associated with the surgery. Though the results of the three studies were promising, the overall quality of the studies was very low. More research is needed to demonstrate the efficacy and safety of this treatment and better understand its mechanism for treating this disorder. Conclusion: DBS may play an important role in treating anorexia nervosa that has proven refractory to standard treatments. DBS may be associated with improvement in physical symptoms, psychological outcomes, and changes in brain glucose metabolism, but more research is needed to further examine this association. Keywords: deep brain stimulation, anorexia nervosa, femal

Competing Phases, Strong Electron-Phonon Interaction and Superconductivity in Elemental Calcium under High Pressure

The observed "simple cubic" (sc) phase of elemental Ca at room temperature in the 32-109 GPa range is, from linear response calculations, dynamically unstable. By comparing first principle calculations of the enthalpy for five sc-related (non-close-packed) structures, we find that all five structures compete energetically at room temperature in the 40-90 GPa range, and three do so in the 100-130 GPa range. Some competing structures below 90 GPa are dynamically stable, i.e., no imaginary frequency, suggesting that these sc-derived short-range-order local structures exist locally and can account for the observed (average) "sc" diffraction pattern. In the dynamically stable phases below 90 GPa, some low frequency phonon modes are present, contributing to strong electron-phonon (EP) coupling as well as arising from the strong coupling. Linear response calculations for two of the structures over 120 GPa lead to critical temperatures in the 20-25 K range as is observed, and do so without unusually soft modes.Comment: 8 pages, 6 figures, 1 table, accepted for publication in Phys. Rev.

Mott transition in anharmonic confinement

Two effects are identified that affect the visibility of the Mott transition in an atomic gas in an optical lattice confined in a power-law potential. The transition can be made more pronounced by increasing the power law, but at the same time, experimental uncertainty in the number of particles will induce corresponding fluctuations in the measured condensate fraction. Calculations in two dimensions indicate that a potential slightly more flat-bottomed than a quadratic one is to be preferred for a wide range of particle number fluctuation size.Comment: 4 pages, 4 figure

Local density of states in the vortex lattice in a type II superconductor

Local density of states (LDOS) in the triangular vortex lattice is investigated based on the quasi-classical Eilenberger theory. We consider the case of an isotropic s-wave superconductor with the material parameter appropriate to NbSe_2. At a weak magnetic field, the spatial variation of the LDOS shows cylindrical structure around a vortex core. On the other hand, at a high field where the core regions substantially overlap each other, the LDOS is sixfold star-shaped structure due to the vortex lattice effect. The orientation of the star coincides with the experimental data of the scanning tunneling microscopy. That is, the ray of the star extends toward the nearest-neighbor (next nearest-neighbor) vortex direction at higher (lower) energy.Comment: 10 pages, RevTex, 32 figure

SO(5) theory of insulating vortex cores in high-TcT_c materials

We study the fermionic states of the antiferromagnetically ordered vortex cores predicted to exist in the superconducting phase of the newly proposed SO(5) model of strongly correlated electrons. Our model calculation gives a natural explanation of the recent STM measurements on BSCCO, which in surprising contrast to YBCO revealed completely insulating vortex cores.Comment: 4 pages, 1 figur

A Self-Consistent Microscopic Theory of Surface Superconductivity

The electronic structure of the superconducting surface sheath in a type-II superconductor in magnetic fields $H_{c2}<H<H_{c3}$ is calculated self-consistently using the Bogoliubov-de Gennes equations. We find that the pair potential $\Delta(x)$ exhibits pronounced Friedel oscillations near the surface, in marked contrast with the results of Ginzburg-Landau theory. The role of magnetic edge states is emphasized. The local density of states near the surface shows a significant depletion near the Fermi energy due to the development of local superconducting order. We suggest that this structure could be unveiled by scanning-tunneling microscopy studies performed near the edge of a superconducting sample.Comment: 12 pages, Revtex 3.0, 3 postscript figures appende

Real-space local polynomial basis for solid-state electronic-structure calculations: A finite-element approach

We present an approach to solid-state electronic-structure calculations based on the finite-element method. In this method, the basis functions are strictly local, piecewise polynomials. Because the basis is composed of polynomials, the method is completely general and its convergence can be controlled systematically. Because the basis functions are strictly local in real space, the method allows for variable resolution in real space; produces sparse, structured matrices, enabling the effective use of iterative solution methods; and is well suited to parallel implementation. The method thus combines the significant advantages of both real-space-grid and basis-oriented approaches and so promises to be particularly well suited for large, accurate ab initio calculations. We develop the theory of our approach in detail, discuss advantages and disadvantages, and report initial results, including the first fully three-dimensional electronic band structures calculated by the method.Comment: replacement: single spaced, included figures, added journal referenc

Basis Functions for Linear-Scaling First-Principles Calculations

In the framework of a recently reported linear-scaling method for density-functional-pseudopotential calculations, we investigate the use of localized basis functions for such work. We propose a basis set in which each local orbital is represented in terms of an array of `blip functions'' on the points of a grid. We analyze the relation between blip-function basis sets and the plane-wave basis used in standard pseudopotential methods, derive criteria for the approximate equivalence of the two, and describe practical tests of these criteria. Techniques are presented for using blip-function basis sets in linear-scaling calculations, and numerical tests of these techniques are reported for Si crystal using both local and non-local pseudopotentials. We find rapid convergence of the total energy to the values given by standard plane-wave calculations as the radius of the linear-scaling localized orbitals is increased.Comment: revtex file, with two encapsulated postscript figures, uses epsf.sty, submitted to Phys. Rev.