757 research outputs found
Assessing the accuracy of quantum Monte Carlo and density functional theory for energetics of small water clusters
We present a detailed study of the energetics of water clusters (HO)
with , comparing diffusion Monte Carlo (DMC) and approximate density
functional theory (DFT) with well converged coupled-cluster benchmarks. We use
the many-body decomposition of the total energy to classify the errors of DMC
and DFT into 1-body, 2-body and beyond-2-body components. Using both
equilibrium cluster configurations and thermal ensembles of configurations, we
find DMC to be uniformly much more accurate than DFT, partly because some of
the approximate functionals give poor 1-body distortion energies. Even when
these are corrected, DFT remains considerably less accurate than DMC. When both
1- and 2-body errors of DFT are corrected, some functionals compete in accuracy
with DMC; however, other functionals remain worse, showing that they suffer
from significant beyond-2-body errors. Combining the evidence presented here
with the recently demonstrated high accuracy of DMC for ice structures, we
suggest how DMC can now be used to provide benchmarks for larger clusters and
for bulk liquid water.Comment: 34 pages, 6 figure
Continuum variational and diffusion quantum Monte Carlo calculations
This topical review describes the methodology of continuum variational and
diffusion quantum Monte Carlo calculations. These stochastic methods are based
on many-body wave functions and are capable of achieving very high accuracy.
The algorithms are intrinsically parallel and well-suited to petascale
computers, and the computational cost scales as a polynomial of the number of
particles. A guide to the systems and topics which have been investigated using
these methods is given. The bulk of the article is devoted to an overview of
the basic quantum Monte Carlo methods, the forms and optimisation of wave
functions, performing calculations within periodic boundary conditions, using
pseudopotentials, excited-state calculations, sources of calculational
inaccuracy, and calculating energy differences and forces
Novel Processing of Hydroxyapatite-Zirconia Composites using Nano-Sized Particles
A method to produce composites of hydroxyapatite (HA) with ZrO2 (Y-TZP) additions between 0 and 10 wt % at a low sinter temperature of 750 °C is presented. The method limits the amount of reaction between the HA and ZrO2. A ZrO2 loading level that would improve strength while limiting the decomposition of the HA is studied
Investigation into the Ultrasound Setting of Glass Ionomer Cements. Part I. Postulated Modalities
Fuji IX glass is a low phosphate, low soda containing glass which has strontia incorporated as a radio-pacifier. This glass material was analyzed during exposure to different durations of ultrasound in an attempt to determine how the glass phase reacts. Cross sections of Fuji IX cement, set both conventionally and by ultrasound, were subsequently compared by microscopy. The resultant data were analyzed in detail
Quantum Monte Carlo study of the Ne atom and the Ne+ ion
We report all-electron and pseudopotential calculations of the
ground-stateenergies of the neutral Ne atom and the Ne+ ion using the
variational and diffusion quantum Monte Carlo (DMC) methods. We investigate
different levels of Slater-Jastrow trial wave function: (i) using Hartree-Fock
orbitals, (ii) using orbitals optimized within a Monte Carlo procedure in the
presence of a Jastrow factor, and (iii) including backflow correlations in the
wave function. Small reductions in the total energy are obtained by optimizing
the orbitals, while more significant reductions are obtained by incorporating
backflow correlations. We study the finite-time-step and fixed-node biases in
the DMC energy and show that there is a strong tendency for these errors to
cancel when the first ionization potential (IP) is calculated. DMC gives highly
accurate values for the IP of Ne at all the levels of trial wave function that
we have considered
A CASE STUDY ON THE EFFECTS OF THE UPPER ARM DEFINITION ON SHOULDER AND ELBOW KINEMATICS DURING THE BADMINTON SMASH
The aim of this case study was to identify the effect of the upper arm definition on shoulder and elbow kinematics during the badminton smash. A method was proposed that corrects the false external rotation when using the shoulder-elbow-wrist plane based on the carrying angle (ISB-2C) and compared to ISB recommendations for defining the upper arm (ISB-1 and ISB-2). Differences were found in shoulder and elbow kinematics, in particular angular velocities. Both magnitude and the time history of the angular velocity were affected. In particular, use of the medial and lateral epicondyles was unable to detect a reasonable signal for pronation/supination. ISB-2C reduces some of the problems associated with ISB-1 and ISB-2 e.g. soft-tissue artifact, proximity of medial and lateral epicondyles to the humeral longitudinal axis and false external rotation caused by the carrying angle
Aluminium-Free Glass Polyalkenoate Spinal Cements
Addition of Yitanium increased the Wt & St and the sf strength of these cements compared to the control (BT 101). © 2011 IEEE
Degradable Borate Glass Polyalkenoate Cements
Glass polyalkenoate cements (GPCs) containing aluminum-free borate glasses having the general composition Ag2O-Na2O-CaO-SrO-ZnO- TiO2-B2O3 were evaluated in this work. An initial screening study of sixteen compositions was used to identify regions of glass formation and cement compositions with promising rheological properties. The results of the screening study were used to develop four model borate glass compositions for further study. A second round of rheological experiments was used to identify a preferred GPC formulation for each model glass composition. The model borate glasses containing higher levels of TiO2 (7.5 mol %) tended to have longer working times and shorter setting times. Dissolution behavior of the four model GPC formulations was evaluated by measuring ion release profiles as a function of time. All four GPC formulations showed evidence of incongruent dissolution behavior when considering the relative release profiles of sodium and boron, although the exact dissolution profile of the glass was presumably obscured by the polymeric cement matrix. Compression testing was undertaken to evaluate cement strength over time during immersion in water. The cements containing the borate glass with 7.5 mol % TiO2 had the highest initial compressive strength, ranging between 20 and 30 MPa. No beneficial aging effect was observed-instead, the strength of all four model GPC formulations was found to degrade with time. © 2014 Springer Science+Business Media
Comparison of an Experimental Bone Cement with a Commercial Control, Hydroset™
Glass polyalkenoate cements based on strontium calcium zinc silicate glasses (Zn-GPCs) and high molecular weight polyacrylic acids (PAA) (MW; 52,000-210,000) have been shown to exhibit mechanical properties and in vitro bioactivity suitable for arthroplasty applications. Unfortunately, these formulations exhibit working times and setting times which are too short for invasive surgical applications such as bone void filling and fracture fixation. In this study, Zn-GPCs were formulated using a low molecular weight PAA (MW; 12,700) and a modifying agent, trisodium citrate dihydrate (TSC), with the aim of improving the rheological properties of Zn-GPCs. These novel formulations were then compared with commercial self-setting calcium phosphate cement, Hydroset™, in terms of compressive strength, biaxial flexural strength and Young\u27s modulus, as well as working time, setting time and injectability. The novel Zn-GPC formulations performed well, with prolonged mechanical strength (39 MPa, compression) greater than both vertebral bone (18.4 MPa) and the commercial control (14 MPa). However, working times (2 min) and rheological properties of Zn-GPCs, though improved, require further modifications prior to their use in minimally invasive surgical techniques. © 2009 Springer Science+Business Media, LLC
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