84 results found
Mason DR, Race CP, Foo MHF, et al., 2012, Resonant charging and stopping power of slow channelling atoms in a crystalline metal, NEW JOURNAL OF PHYSICS, Vol: 14, ISSN: 1367-2630
Spencer JS, Blunt NS, Foulkes WMC, 2012, The sign problem and population dynamics in the full configuration interaction quantum Monte Carlo method, JOURNAL OF CHEMICAL PHYSICS, Vol: 136, ISSN: 0021-9606
Mason DR, Race CP, Foulkes WMC, et al., 2011, Quantum mechanical simulations of electronic stopping in metals, Nucl. Instrum. Meth. Phys. Res. B, Vol: 269, Pages: 1640-1645, ISSN: 0168-583X
Race CP, Mason DR, Finnis MW, et al., 2010, The treatment of electronic excitations in atomistic models of radiation damage in metals, REPORTS ON PROGRESS IN PHYSICS, Vol: 73, ISSN: 0034-4885
McEniry EJ, Wang Y, Dundas D, et al., 2010, Modelling non-adiabatic processes using correlated electron-ion dynamics, EUROPEAN PHYSICAL JOURNAL B, Vol: 77, Pages: 305-329, ISSN: 1434-6028
Race CP, Mason DR, le Page J, et al., 2010, Aiding the design of radiation resistant materialswith multiphysics simulations of damage processes, 2009 MRS Fall Meeting, Publisher: Cambridge University Press
Mason DR, Foulkes WMC, Sutton AP, 2010, A simple model for large-scale simulations of fcc metals with explicit treatment of electrons, PHILOSOPHICAL MAGAZINE LETTERS, Vol: 90, Pages: 51-60, ISSN: 0950-0839
le PJ, Mason DR, Race CP, et al., 2009, How good is damped molecular dynamics as a method to simulate radiation damage in metals?, NEW J PHYS, Vol: 11, Pages: 1-15, ISSN: 1367-2630
Classical molecular dynamics ( MD) is a frequently used technique in the study of radiation damage cascades because it provides information on very small time and length scales inaccessible to experiment. In a radiation damage process, energy transfer from ions to electrons may be important, yet there is continued uncertainty over how to accurately incorporate such effects in MD. We introduce a new technique based on the quantum mechanical Ehrenfest approximation to evaluate different methods of accounting for electronic losses. Our results suggest that a damping force proportional to velocity is sufficient to model energy transfer from ions to electrons in most low energy cascades. We also find, however, that a larger rate of energy transfer is seen when the ionic kinetic energy is confined to a focused sequence of collisions. A viscous damping coefficient dependent on the local atomic environment is shown to be an excellent model for electronic energy losses in low energy cascades in metals.
Hine NDM, Frensch K, Foulkes WMC, et al., 2009, Supercell size scaling of density functional theory formation energies of charged defects, PHYSICAL REVIEW B, Vol: 79, ISSN: 2469-9950
Le Page J, Mason DR, Foulkes WMC, 2008, The Ehrenfest approximation for electrons coupled to a phonon system, J. Phys.: Condens. Matter, Vol: 20, Pages: 125212-1-125212-13
Drummond ND, Needs RJ, Sorouri A, et al., 2008, Finite-size errors in continuum quantum Monte Carlo calculations, Phys. Rev. B, Vol: 78
Horsfield AP, Finnis M, Foulkes M, et al., 2008, Correlated electron-ion dynamics in metallic systems, Computational Materials Science, Vol: 44, Pages: 16-20
Wood B, Hine NDM, Foulkes WMC, et al., 2007, Quantum Monte Carlo calculations of the surface energy of an electron gas, PHYS REV B, Vol: 76
We present quantum Monte Carlo calculations of the surface energy of the electron gas jellium . Our results agree with the best estimates obtained by other methods, thus appearing to resolve the controversy which currently exists and paving the way for future simulations of real surface systems.
Mason DR, Le Page J, Race C, et al., 2007, Electronic damping of atomic dynamics in irradiation damage of metals, J. Phys.: Condens. Matter, Vol: 19, Pages: 436209-1-436209-13
Hine NDM, Foulkes WMC, 2007, Localization lengths over metal to band insulator transitions, J. Phys.: Condens. Matter, Vol: 19, ISSN: 0953-8984
Wood B, Foulkes WMC, 2006, Improved many-electron wavefunctions from plasmon normal modes, JOURNAL OF PHYSICS-CONDENSED MATTER, Vol: 18, Pages: 2305-2326, ISSN: 0953-8984
Sorouri A, Foulkes WMC, Hine NDM, 2006, Accurate and efficient method for the treatment of exchange in a plane-wave basis, JOURNAL OF CHEMICAL PHYSICS, Vol: 124, ISSN: 0021-9606
Wood B, Foulkes WMC, Towler MD, et al., 2004, Coulomb finite-size effects in quasi-two-dimensional systems, JOURNAL OF PHYSICS-CONDENSED MATTER, Vol: 16, Pages: 891-902, ISSN: 0953-8984
Nekovee M, Foulkes WMC, Needs RJ, 2003, Quantum Monte Carlo investigations of density functional theory of the strongly inhomogeneous electron gas, Physical Review B, Vol: 68, ISSN: 1550-235X
We use a variational quantum Monte Carlo realization of the adiabatic connection technique to calculate the most relevant quantities in Hohenberg-Kohn-Sham density functional theory for several strongly inhomogeneous electron-gas systems. Results for the coupling-constant dependence of the exchange-correlation energy, the pair-correlation function, the exchange-correlation hole, and the exchange and correlation energy densities are presented. Comparisons are made with the interaction strength interpolation (ISI) approximation, the local density approximation (LDA), the gradient expansion approximation (GEA), the generalized gradient approximation (GGA), and the weighted density approximation (WDA). The coupling-constant dependence of the exchange-correlation energy is accurately described by an ISI model that incorporates information on the strong-interaction limit. Unlike either the LDA or GEA, the WDA is successful in describing the nonlocal structure of the exchange-correlation hole. The LDA errors in the exchange-correlation energy density show a remarkable correlation with the Laplacian of the density. The GGA worsens the error in the integrated exchange-correlation energy as the inhomogeneity of the systems increases. This failure is shared by current meta-GGA functionals and is shown to be caused by the inability of these functionals to describe the LDA overestimation (in absolute value) of the exchange energy density around density maxima. It is suggested that this effect could be taken into account by including Laplacian terms in semilocal density functionals.
Nekovee M, WMC F, Needs RJ, 2003, Quantum Monte Carlo studies of density functional theory, MATHEMATICS AND COMPUTERS IN SIMULATION, Vol: 62, Pages: 463-470, ISSN: 0378-4754
We review our variational quantum Monte Carlo (VMC) approach for evaluating the key quantities in the density functional theory of inhomogeneous many-electron systems, and describe the underlying simulation algorithm and its parallel implementation. We discuss the insights gained from our recent application of the method to the study of the density functional theory (DFT) of the strongly inhomogeneous electron gas. (C) 2003 IMACS. Published by Elsevier Science B.V. All rights reserved.
Gaudoin R, WMC F, Rajagopal G, 2002, Ab initio calculations of the cohesive energy and the bulk modulus of aluminium, J PHYS-CONDENS MAT, Vol: 14, Pages: 8787-8793, ISSN: 0953-8984
To date there have been few attempts to calculate bulk properties such as the cohesive energy or the bulk modulus of metals using Monte Carlo (MC) methods. We present a variational MC calculation for aluminium and find that methods used to deal with finite-size effects work just as well as for insulators, despite the presence of a Fermi surface. However, the large statistical uncertainties are a problem when evaluating the bulk modulus.
Gaudoin R, Foulkes WMC, 2002, Ab initio calculations of bulk moduli and comparison with experiment, PHYSICAL REVIEW B, Vol: 66, ISSN: 2469-9950
Nekovee M, Foulkes WMC, Needs RJ, 2001, Quantum Monte Carlo analysis of exchange and correlation in the strongly inhomogeneous electron gas, PHYSICAL REVIEW LETTERS, Vol: 87, ISSN: 0031-9007
Gaudoin R, Nekovee M, Foulkes WMC, et al., 2001, Inhomogeneous random-phase approximation and many-electron trial wave functions, Phys. Rev. B, Vol: 63, ISSN: 1098-0121
The long-range electronic correlations in a uniform electron gas may be deduced from the random-phase approximation (RPA) of Bohm and Pines [Phys. Rev. 92, 609 (1953)]. Here we generalize the RPA to nonuniform systems and use it to derive many-electron Slater-Jastrow trial wave functions for quantum Monte Carlo simulations. The RPA theory fixes the long-range behavior of the inhomogeneous two-body terms in the Jastrow factor and provides an accurate analytic expression for the one-body terms. It also explains the success of Slater-Jastrow trial functions containing determinants of Hartree-Fock or density-functional orbitals, even though these theories do not include Jastrow factors. After adjusting the RPA Jastrow factor to incorporate the known short-range behavior, we test it using variational Monte Carlo simulations. In the small inhomogeneous electron gas system we consider, the analytic RPA-based Jastrow factor slightly outperforms the standard numerically optimized form. The inhomogeneous RPA theory therefore enables us to reduce or even avoid the costly numerical optimization process.
This article describes the variational and fixed-node diffusion quantum Monte Carlo methods and how they may be used to calculate the properties of many-electron systems. These stochastic wave-function-based approaches provide a very direct treatment of quantum many-body effects and serve as benchmarks against which other techniques may be compared. They complement the less demanding density-functional approach by providing more accurate results and a deeper understanding of the physics of electronic correlation in real materials. The algorithms are intrinsically parallel, and currently available high-performance computers allow applications to systems containing a thousand or more electrons. With these tools one can study complicated problems such as the properties of surfaces and defects, while including electron correlation effects with high precision. The authors provide a pedagogical overview of the techniques and describe a selection of applications to ground and excited states of solids and clusters.
WMC F, Hood RQ, Needs RJ, 1999, Symmetry constraints and variational principles in diffusion quantum Monte Carlo calculations of excited-state energies, Physical Review B, Vol: 60, ISSN: 1550-235X
Fixed-node diffusion Monte Carlo (DMC) is a stochastic algorithm for finding the lowest energy many-fermion wave function with the same nodal surface as a chosen trial function. It has proved itself among the most accurate methods available for calculating many-electron,ground states, and is one of the few approaches that can be applied to systems large enough to act as realistic models of solids. In attempts to use fixed-node DMC for excited-state calculations, it has often been assumed that the DMC energy must be greater than or equal to the energy of the lowest exact eigenfunction with the same symmetry as the trial function. We show that this assumption is not justified unless the trial function transforms according to a one-dimensional irreducible representation of the symmetry group of the Hamiltonian. If the trial function transforms according to a multidimensional irreducible representation, corresponding to a degenerate energy level, the DMC energy may lie below the energy of the lowest eigenstate of that symmetry. Weaker variational bounds may then be obtained by choosing trial functions transforming according to one-dimensional irreducible representations of subgroups of the full symmetry group. [S0163-1829(99)09331-5].
PRC K, Hood RQ, Williamson AJ, et al., 1999, Finite-size errors in quantum many-body simulations of extended systems, PHYS REV B, Vol: 59, Pages: 1917-1929, ISSN: 0163-1829
Further developments are introduced in the theory of finite-size errors in quantum many-body simulations of extended systems using periodic boundary conditions. We show that our recently introduced model periodic Coulomb interaction [A. J. Williamson et al., Phys. Rev. B 55, R4851 (1997)] can be applied consistently to all Coulomb interactions in the system. The model periodic Coulomb interaction greatly reduces the finite-size errors in quantum many-body simulations. We illustrate the practical application of our techniques with Hartree-Fock and variational and diffusion quantum Monte Carlo calculations for ground- and excited-state calculations. We demonstrate that the finite-size effects in electron promotion and electron addition/subtraction excitation energy calculations are very similar. [S0163-1829(99)07303-8].
Foulkes WMC, Nekovee M, Gaudoin RL, et al., 1999, Quantum Monte Carlo simulations of real solids, High Performance Computing, Editors: Allen, Guest, Simpson, Henty, Nicole, Publisher: Kluwer Academic/Plenum, Pages: 165-174
Nekovee M, Foulkes WMC, Williamson AJ, et al., 1999, A quantum Monte Carlo approach to the adiabatic connection method, Adv. Quantum Chem., Vol: 33, Pages: 189-207
Stedman ML, WMC F, 1998, Talus - A quantum Monte Carlo modelling suite, COMPUT PHYS COMMUN, Vol: 113, Pages: 180-198, ISSN: 0010-4655
We present the Talus Quantum Monte Carlo modelling suite of programs. The suite provides an extensible environment for the study of a diverse range of physical systems with several QMC algorithms. The overriding concern behind the design of the suite has been to enable new algorithms and, in particular, new systems to be studied with minimal implementation and testing overhead. To this end, we have designed the Monte Carlo and measurement functions to be independent of the actual wavefunction. We describe the main algorithms used, discuss the unusual features of the suite's implementation, and provide results for a simple test calculation. (C) 1998 Published by Elsevier Science B.V.
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