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 (H$_2$O)$_n$ with $n \le 6$, 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

The Influence of Ultrasonic Setting on Fluoride Release from Glass Polyalkenoate Cements

The effects of ultrasonic setting on fluoride release from glass polyalkenoate cements (GPC) were investigated. Cements based on two different aluminosilicate glasses that contained silica, alumina, phosphate, and strontia were used for the experiment. A series of GPCs were examined to show that increased release F- ion occurs, regardless of GPC composition. The results show that there is a greater release of fluoride from the ultrasonically set samples than those set chemically, regardless of the chemical composition. The snap set caused by the ultrasound is likely to be due to a combination of cavitation, improved mixing of the constituents, and better compaction. A reduction in mean particle size was observed, which may be due to the breaking up of agglomerates of particles

The Processing, Mechanical Properties and Bioactivity of Zinc based Glass Ionomer Cements

The suitability of Glass Ionomer Cements (GICs) for use in orthopaedics is retarded by the presence in the glass phase of aluminium, a neurotoxin. Unfortunately, the aluminium ion plays an integral role in the setting process of a GIC and its absence is likely to hinder cement formation. However, zinc oxide, a bacteriocide, can act both as a network modifying oxide and an intermediate oxide in a similar fashion to alumina and so ternary systems based on zinc silicates often have extensive regions of glass formation. The purpose of this research was to produce novel GICs based on calcium zinc silicate glasses and to evaluate their rheological, mechanical and biocompatible properties with the ultimate objective of developing a new range of cements for skeletal applications. The work reported shows that GICs based on two different glasses, A and B (0.05CaO · 0.53ZnO · 0.42SiO2 and 0.14CaO · 0.29ZnO · 0.57SiO2, respectively), exhibited handling properties and flexural strengths comparable to conventional GICs. Upon immersion in simulated body fluid of a GIC based on glass B, an amorphous calcium phosphate layer nucleated on the surface of the cement indicating that these cements are bioactive in nature. © 2005 Springer Science + Business Media, Inc

A Novel Tantalum-Containing Bioglass. Part I. Structure and Solubility

Bio glasses are employed for surgical augmentation in a range of hard tissue applications. Tantalum is a bioactive and biocompatible transition metal that has been used as an orthopedic medical device. It has a range of biological and physical properties that make its incorporation into ionic form into bioactive glass systems promising for various clinical applications. The work herein reports the characterization and properties of novel tantalum-containing glasses. A series of glasses based on the system 48SiO2-(36-X)ZnO-6CaO-8SrO-2P2O5-XTa2O5 with X varying from 0 mol% (TA0) to 0.5 mol% (TA2) were synthesized. The addition of small amounts of Ta2O5 did not cause crystallization of the glasses but increasing Ta2O5 content at the expense of ZnO was found to result in an increased number of bridging oxygens (BOs). This, along with the data recorded by differential thermal analysis (DTA) and magic angle spinning-nuclear magnetic resonance (MAS-NMR), confirms that Ta acts as a glass former in this series. Solubility experiments showed that minor changes in the glass structure caused by Ta incorporation (0.5 mol%) exhibited greater cumulative % weight loss, pH values and cumulative Zn2+ and Sr2+ ion concentration over a period of 30 days of maturation, when compared to Ta2O5-free glasses. The results presented in this article confirm that replacing ZnO with Ta2O5 in silicate glasses results in the formation of stronger bonds within the glass network without any adverse effects on the solubility of the glasses prepared from them

Materials and Techniques Used in Cranioplasty Fixation: A Review

Cranioplasty is the surgical repair of a deficiency or deformity of the skull. The purpose of cranioplasty is to provide protection for the brain following cranial surgery, and to offer relief to psychological disadvantages while increasing social performance. There are several materials that had been used for cranioplasty, but an ideal product has yet to be developed, hence the ongoing research into biologic and non-biologic alternatives to the existing materials. This article critiques the products currently used for cranioplasty in order to facilitate the development of new materials, which can improve patient outcomes

RNA-seq reveals post-transcriptional regulation of Drosophila insulin-like peptide dilp8 and the neuropeptide-like precursor Nplp2 by the exoribonuclease Pacman/XRN1

Ribonucleases are critically important in many cellular and developmental processes and defects in their expression are associated with human disease. Pacman/XRN1 is a highly conserved cytoplasmic exoribonuclease which degrades RNAs in a 5' - 3' direction. In Drosophila, null mutations in pacman result in small imaginal discs, a delay in onset of pupariation and lethality during the early pupal stage. In this paper, we have used RNA-seq in a genome-wide search for mRNAs misregulated in pacman null wing imaginal discs. Only 4.2% of genes are misregulated ±>2-fold in pacman null mutants compared to controls, in line with previous work showing that Pacman has specificity for particular mRNAs. Further analysis of the most upregulated mRNAs showed that Pacman post-transcriptionally regulates the expression of the secreted insulin-like peptide Dilp8. Dilp8 is related to human IGF-1, and has been shown to co-ordinate tissue growth with developmental timing in Drosophila. The increased expression of Dilp8 is consistent with the developmental delay seen in pacman null mutants. Our analysis, together with our previous results, show that the normal role of this exoribonuclease in imaginal discs is to suppress the expression of transcripts that are crucial in apoptosis and growth control during normal development

Comparison of Failure Mechanisms for Cements Used in Skeletal Luting Applications

Glass Polyalkenoate Cements (GPCs) based on strontium calcium zinc silicate (Sr-Ca-Zn-SiO2) glasses and low molecular weight poly (acrylic acid) (PAA) have been shown to exhibit suitable compressive strength (65 MPa) and flexural strength (14 MPa) for orthopaedic luting applications. In this study, two such GPC formulations, alongside two commercial cements (Simplex ® P and Hydroset™) were examined. Fracture toughness and tensile bond strength to sintered hydroxyapatite and a biomedical titanium alloy were examined. Fracture toughness of the commercial Poly(methyl methacrylate) cement, Simplex® P, (3.02 MPa m1/2) was superior to that of the novel GPC (0.36 MPa m1/2) and the commercial calcium phosphate cement, Hydroset™, for which no significant fracture toughness was obtained. However, tensile bond strengths of the novel GPCs (0.38 MPa), after a prolonged period (30 days), were observed to be superior to commercial controls (Simplex™ P: 0.07 MPa, Hydroset™: 0.16 MPa). © 2009 Springer Science+Business Media, LLC

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