585 research outputs found
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
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
Transition metal materials: a first principles approach to the electronic structure of the insulating phase
Recent progress in the application of first principles theory to the electronic structure of transition metal materials is reviewed with particular emphasis on the use of the exact exchange interaction. The success of this approach is exemplified by calculations on a range of materials: simple monoxides, chromium cyanides and perovskite structure copper fluorides. The reliability of computed properties is established for lattice structures, spin-couplings, spin-lattice interactions, orbital ordering effects and the changes in the ground state induced by hole doping.</p
Jastrow correlation factor for atoms, molecules, and solids
A form of Jastrow factor is introduced for use in quantum Monte Carlo
simulations of finite and periodic systems. Test data are presented for atoms,
molecules, and solids, including both all-electron and pseudopotential atoms.
We demonstrate that our Jastrow factor is able to retrieve a large fraction of
the correlation energy
Diamond and -tin structures of Si studied with quantum Monte Carlo calculations
We have used diffusion quantum Monte Carlo (DMC) calculations to study the
pressure-induced phase transition from the diamond to -tin structure in
silicon. The calculations employ the pseudopotential technique and
systematically improvable B-spline basis sets. We show that in order to achieve
a precision of 1 GPa in the transition pressure the non-cancelling errors in
the energies of the two structures must be reduced to 30 meV/atom. Extensive
tests on system size errors, non-local pseudopotential errors, basis-set
incompleteness errors, and other sources of error, performed on periodically
repeated systems of up to 432 atoms, show that all these errors together can be
reduced to well below 30 meV/atom. The calculated DMC transition pressure is
about 3-4 GPa higher than the accepted experimental range of values, and we
argue that the discrepancy may be due to the fixed-node error inherent in DMC
techniques.Comment: 10 pages, 4 figure
All-electron quantum Monte Carlo calculations for the noble gas atoms He to Xe
We report all-electron variational and diffusion quantum Monte Carlo (VMC and
DMC) calculations for the noble gas atoms He, Ne, Ar, Kr, and Xe. The
calculations were performed using Slater-Jastrow wave functions with
Hartree-Fock single-particle orbitals. The quality of both the optimized
Jastrow factors and the nodal surfaces of the wave functions declines with
increasing atomic number Z, but the DMC calculations are tractable and well
behaved in all cases. We discuss the scaling of the computational cost of DMC
calculations with Z
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