Publications
102 results found
Lim A, Foulkes WM, Horsfield AP, et al., 2016, Electron elevator: excitations across the band gap via a dynamical gap state, Physical Review Letters, Vol: 116, Pages: 1-1, ISSN: 0031-9007
We use time-dependent density functional theory to study self-irradiated Si. We calculate the electronic stopping power of Si in Si by evaluating the energy transferred to the electrons per unit path length by an ion of kinetic energy from 1 eV to 100 keV moving through the host. Electronic stopping is found to be significant below the threshold velocity normally identified with transitions across the band gap. A structured crossover at low velocity exists in place of a hard threshold. An analysis of the time dependence of the transition rates using coupled linear rate equations enables one of the excitation mechanisms to be clearly identified: a defect state induced in the gap by the moving ion acts like an elevator and carries electrons across the band gap.
Spencer JS, Blunt NS, Vigor WA, et al., 2015, Open-source development experiences in scientific software: the HANDE quantum Monte Carlo project, Journal of Open Research Software, Vol: 3, ISSN: 2049-9647
The HANDE quantum Monte Carlo project offers accessible stochastic algorithmsfor general use for scientists in the field of quantum chemistry. HANDE is anambitious and general high-performance code developed by ageographically-dispersed team with a variety of backgrounds in computationalscience. In the course of preparing a public, open-source release, we havetaken this opportunity to step back and look at what we have done and what wehope to do in the future. We pay particular attention to development processes,the approach taken to train students joining the project, and how a flathierarchical structure aids communication
Edmunds DM, Tangney P, Vvedensky DD, et al., 2015, Free-energy coarse-grained potential for C60, Journal of Chemical Physics, Vol: 143, Pages: 164509-1-164509-4, ISSN: 0021-9606
We propose a new deformable free energy method for generating a free-energy coarse-graining potential for C60. Potentials generated from this approach exhibit a strong temperature dependence and produce excellent agreement with benchmark fully atomistic molecular dynamics simulations. Parameter sets for analytical fits to this potential are provided at four different temperatures.
Guhl H, Lee HS, Tangney P, et al., 2015, Structural and electronic properties of sigma7 grain boundaries in alpha-Al2O3, Acta Materialia, Vol: 99, Pages: 16-28, ISSN: 1359-6454
Applying simulated annealing with a classical potential followed by screening of low-energy structures with density functional theory, we examined the atomic and electronic structures of the View the MathML source and View the MathML source symmetric tilt grain boundaries in α-Al2O3. The lowest energy View the MathML source boundary exhibits a pronounced pattern of alternating columns of exclusively four- or fivefold coordinated Al atoms, with a grain boundary energy of 1.84 Jm−2. For the View the MathML source boundary, numerous structures were found with energy just below 2.11 Jm−2. Furthermore, by analysing the full set of candidate structures generated by simulated annealing for the two grain boundaries, we find that the number of fivefold coordinated Al atoms tends to increase with grain boundary energy, which we can also correlate with the behaviour of the electronic density of states. On the other hand, we find no systematic trend with energy that might be expected for other quantities, notably the excess volume of the interface. We compare simulated high-resolution transmission electron microscope (HRTEM) images of the lowest energy calculated structures with experimental images. The disparate structural and electronic features of these two boundaries suggest reasons for their very different oxygen diffusion coefficients that have been observed experimentally.
Malone FD, Blunt NS, Shepherd JJ, et al., 2015, Interaction picture density matrix quantum Monte Carlo, Journal of Chemical Physics, Vol: 143, ISSN: 1089-7690
The recently developed density matrix quantum Monte Carlo (DMQMC) algorithm stochastically samplesthe N-body thermal density matrix and hence provides access to exact properties of many-particle quantumsystems at arbitrary temperatures. We demonstrate that moving to the interaction picture provides substan-tial benefits when applying DMQMC to interacting fermions. In this first study, we focus on a system ofmuch recent interest: the uniform electron gas in the warm dense regime. The basis set incompleteness errorat finite temperature is investigated and extrapolated via a simple Monte Carlo sampling procedure. Finally,we provide benchmark calculations for a four-electron system, comparing our results to previous work wherepossible.
Azadi S, Foulkes WMC, 2015, Systematic study of finite-size effects in quantum Monte Carlo calculations of real metallic systems, Journal of Chemical Physics, Vol: 143, ISSN: 1089-7690
We present a systematic and comprehensive study of finite-size effects in diffusion quantum Monte Carlo calculations of metals. Several previously introduced schemes for correcting finite-size errors are compared for accuracy and efficiency, and practical improvements are introduced. In particular, we test a simple but efficient method of finite-size correction based on an accurate combination of twist averaging and density functional theory. Our diffusion quantum Monte Carlo results for lithium and aluminum, as examples of metallic systems, demonstrate excellent agreement between all of the approaches considered.
Blein S, Bardel C, Danjean V, et al., 2015, An original phylogenetic approach identified mitochondrial haplogroup T1a1 as inversely associated with breast cancer risk in <i>BRCA2</i> mutation carriers, BREAST CANCER RESEARCH, Vol: 17, ISSN: 1465-5411
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- Citations: 26
Drummond ND, Needs RJ, Sorouri A, et al., 2014, Erratum: Finite-size errors in continuum quantum Monte Carlo calculations (Physical Review B - Condensed Matter and Materials Physics (2008) 78 (125106)), Physical Review B - Condensed Matter and Materials Physics, Vol: 90, ISSN: 1098-0121
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- Citations: 1
Blunt NS, Rogers TW, Spencer JS, et al., 2014, Density-matrix quantum Monte Carlo method, Physical Review B, Vol: 89, ISSN: 2469-9950
We present a quantum Monte Carlo method capable of sampling the full density matrix of a many-particle system at finite temperature. This allows arbitrary reduced density matrix elements and expectation values of complicated nonlocal observables to be evaluated easily. The method resembles full configuration interaction quantum Monte Carlo but works in the space of many-particle operators instead of the space of many-particle wave functions. One simulation provides the density matrix at all temperatures simultaneously, from T=∞ to T=0, allowing the temperature dependence of expectation values to be studied. The direct sampling of the density matrix also allows the calculation of some previously inaccessible entanglement measures. We explain the theory underlying the method, describe the algorithm, and introduce an importance-sampling procedure to improve the stochastic efficiency. To demonstrate the potential of our approach, the energy and staggered magnetization of the isotropic antiferromagnetic Heisenberg model on small lattices, the concurrence of one-dimensional spin rings, and the Renyi S2 entanglement entropy of various sublattices of the 6×6 Heisenberg model are calculated. The nature of the sign problem in the method is also investigated.
Azadi S, 2014, Dissociation of High-Pressure Solid Molecular Hydrogen: A Quantum Monte Carlo and Anharmonic Vibrational Study, Phys. Rev. Lett, Vol: 112
Azadi S, Foulkes WMC, Kuehne TD, 2013, Quantum Monte Carlo study of high pressure solid molecular hydrogen, NEW JOURNAL OF PHYSICS, Vol: 15, ISSN: 1367-2630
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- Citations: 43
Azadi S, Foulkes WMC, 2013, Fate of density functional theory in the study of high-pressure solid hydrogen (vol 88, 014115, 2013), PHYSICAL REVIEW B, Vol: 88, ISSN: 1098-0121
Heuer AH, Nakagawa T, Azar MZ, et al., 2013, On the Growth of Al_2 O_3 Scales, Acta Materialia, Vol: 61, Pages: 6670-6683
Race CP, Mason DR, Foo MHF, et al., 2013, Quantum-classical simulations of the electronic stopping force and charge on slow heavy channelling ions in metals, JOURNAL OF PHYSICS-CONDENSED MATTER, Vol: 25, ISSN: 0953-8984
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- Citations: 16
Kolodrubetz MH, Spencer JS, Clark BK, et al., 2013, The effect of quantization on the full configuration interaction quantum Monte Carlo sign problem, JOURNAL OF CHEMICAL PHYSICS, Vol: 138, ISSN: 0021-9606
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- Citations: 22
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
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- Citations: 15
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
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- Citations: 86
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
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- Citations: 102
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
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- Citations: 28
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
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- Citations: 2
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
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- Citations: 173
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
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
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.
Hine NDM, Foulkes WMC, 2007, Localization lengths over metal to band insulator transitions, J. Phys.: Condens. Matter, Vol: 19, ISSN: 0953-8984
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
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