A Fast and Efficient Algorithm for Slater Determinant Updates in Quantum Monte Carlo Simulations

We present an efficient low-rank updating algorithm for updating the trial wavefunctions used in Quantum Monte Carlo (QMC) simulations. The algorithm is based on low-rank updating of the Slater determinants. In particular, the computational complexity of the algorithm is O(kN) during the k-th step compared with traditional algorithms that require O(N^2) computations, where N is the system size. For single determinant trial wavefunctions the new algorithm is faster than the traditional O(N^2) Sherman-Morrison algorithm for up to O(N) updates. For multideterminant configuration-interaction type trial wavefunctions of M+1 determinants, the new algorithm is significantly more efficient, saving both O(MN^2) work and O(MN^2) storage. The algorithm enables more accurate and significantly more efficient QMC calculations using configuration interaction type wavefunctions

Measurement of hardness and elastic modulus by depth sensing indentation: Improvements to the technique based on continuous stiffness measurement

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Wide dynamic range 2-D nanoindentation: Friction and partial slip at contacts

A new nanomechanical testing system is described. It provides the same force controlled displacement sensing capability as nanoindentation, but now with two completely separated orthogonal axes. Load modulation enables direct determination of contact area and stiffness, both lateral and vertical, along with energy losses from the phase shifts. Two features in particular, wide dynamic ranges of several orders of magnitude of stiffness and a very high degree of mechanical separation (low crosstalk) between the axes, distinguish the technique from AFM. AFM is one of the few techniques to date to investigate tribological single asperity contacts but its mechanical limitations make it difficult to discern the underlying mechanisms. With this new technique, the evolution of a contact under 2-D stresses from deformation-free atomistic scale to initial plasticity along with the associated changes in geometry, can be monitored. Results will be presented showing that unlike in elastic contacts, Mindlin partial slip does not occur immediately under lateral stress in plastically deformed contacts. The evolution of contact area in the initial stages of sliding in the presence of plastic flow will be described, and resembles the predictions of classical Tabor and Johnson models. It will be shown that energy dissipation measured from phase shift of a modulating signal is largely due to interfacial friction rather than volumetric deformation. Prospects for further studies using both shear and normal loading will be discussed

The influence of heat treatment on the microstructural, mechanical and corrosion behaviour of cold sprayed SS 316L coatings

The present study evaluates the response of cold sprayed SS 316L coatings on mild steel substrate to aqueous corrosion in a 0.1 N HNO3 solution as determined using polarization tests. The corrosion behaviour of the SS 316L coating was studied not only in the as-coated condition, but also after heat treatment at 400, 800 and 1,100 °C. Heat treatment reduced the porosity, improved inter-splat bonding, increased the elastic modulus and more importantly increased the corrosion resistance of the cold sprayed SS 316L coating

Improved Dissolution Rate of Piroxicam by Fusion Solid Dispersion Technique

Improving oral bioavailability of drugs those given as solid dosage forms remains a challenge for the formulation scientists due to solubility  problems. The dissolution rate could be the rate-limiting process in the absorption of a drug from a solid dosage form of relatively insoluble drugs. Therefore increase in dissolution of poorly soluble drugs by solid dispersion technique presents a challenge to the formulation scientists. In the present work solid dispersed drug was prepared by Fusion technique as a novel system for enhancing the delivery of piroxicam, a non-steroidal  anti-inflammatory drug. This solid dispersed drug was prepared from polyvinyl pyrrolidone (PVP) (pharmaceutical grade), a biodegradable polymer, to obtain a solution with drug: polymer ratio of 1:5. The release rate of the piroxicam solid dispersed drug was studied in simulated gastric fluid. Fourier transform infrared (FTIR) and scanning electron microscopy (SEM) are used to evaluate the chemical and physical nature. The results showed that the release rates were twice increased in comparison with the pure drug. However, the blend of drug and polymer could be varied to optimize the release rates depending upon the need and formulation

Sub-matrix updates for the Continuous-Time Auxiliary Field algorithm

We present a sub-matrix update algorithm for the continuous-time auxiliary field method that allows the simulation of large lattice and impurity problems. The algorithm takes optimal advantage of modern CPU architectures by consistently using matrix instead of vector operations, resulting in a speedup of a factor of $\approx 8$ and thereby allowing access to larger systems and lower temperature. We illustrate the power of our algorithm at the example of a cluster dynamical mean field simulation of the N\'{e}el transition in the three-dimensional Hubbard model, where we show momentum dependent self-energies for clusters with up to 100 sites

Characterization of the first hexacoordinate phosphorus compound with S→P←S bonds

The first example of a hexacoordinate phosphorus compound [S{6-t-Bu-4-Me-C6H2O}2]2P+(Cl-· C3H4N2) with two S→P bonds is reported. This compound can be construed as an oxophosphonium salt with double intramolecular coordination by sulfur atoms. X-ray structure reveals a facial arrangement of the ligands with two coordinating sulfur atoms cis to each other. The S→P distance of 2.334 (1) Å is one among very short coordinate bond distances between sulfur and phosphorus

Nanoindentation at elevated temperatures

Relating the creep response observed with high temperature instrumented indentation experiments to macroscopic uniaxial creep response is of great practical value. In this review, we present an overview of various methods currently being used to measure creep at small scales with instrumented indentation, with a focus on geometrically self-similar indenters, and their relative merits and demerits from an experimental perspective. A comparison of the various methods to use those instrumented indentation results to predict the uniaxial power law creep response of a wide range of materials (stress exponent of 1 to 8), will be presented to assess their validity. The interplay of size dependent hardness effects, strain rate effects and temperature effects will also be discussed. The extension of rapid testing and mapping techniques to high temperatures will also be demonstrated. Figure 1 shows a map of hardness vs position in a carbide containing steel at 300 degrees C. These techniques are extended to stress exponent and pre-exponential maps determined at high temperatures. Please click Additional Files below to see the full abstract

Mapping of mechanical properties at microstructural length scale in WC-Co cemented carbides: Assessment of hardness and elastic modulus by means of high speed massive nanoindentation and statistical analysis

This paper studies the correlation between the microstructure and the mechanical properties at the nanometric length scale of individual WC grains as well as the metallic cobalt binder in cemented carbide systems. The local crystallographic orientation has been determined by electron backscattered diffraction and the microstructural analysis has been performed using field emission scanning electron microscopy. Small-scale hardness and elastic modulus have been assessed by means of high speed massive nanoindentation and subsequent statistical analysis. The attained mechanical property mappings present a clear correlation between local hardness and stiffness with chemical nature for each constitutive phase as well as with the crystallographic orientation for the WC particles. Besides expected findings associated with individual phases, such as clear anisotropy of the ceramic phase (basal plane being harder and stiffer than the prismatic one) and relatively high flow stress for constrained binder, the protocol implemented provides novel information on local mechanical response at interfaces between ceramic particles with different orientations as well as regions within the metallic cobalt binder close to the WC-Co interface.Peer ReviewedPreprin