73 results found
Mostofi AA, Haynes PD, Skylaris CK, et al., 2007, ONETEP: linear-scaling density-functional theory with plane-waves, MOLECULAR SIMULATION, Pages: 551-555
Conventional methods for atomistic simulations based on density-functional theory (DFT), such as the plane-wave (PW) pseudopotential approach, have had an immense impact on the way in which material properties are studied. In spite of this success, the system-size accessible to such techniques is limited because the algorithms scale with the cube of the number of atoms. The quest to bring to bear the predictive power of DFT calculations on ever larger systems has resulted in much recent interest in linear-scaling methods for DFT simulations. To this end we present an overview of ONETEP (Order-N Total Energy Package), our linear-scaling method based on a PW basis set, which is able to achieve the same accuracy and convergence rate as the conventional PW DFT approach. The novel features of our method which result in its success are described and results of calculations on titanium oxide clusters from the ONETEP parallel code are presented.
Haynes PD, Skylaris CK, Mostofi AA, et al., 2006, ONETEP: linear-scaling density-functional theory with local orbitals and plane waves, PHYS STATUS SOLIDI B, Vol: 243, Pages: 2489-2499, ISSN: 0370-1972
Haynes PD, Skylaris CK, Mostofi AA, et al., 2006, Elimination of basis set superposition error in linear-scaling density-functional calculations with local orbitals optimised in situ, CHEM PHYS LETT, Vol: 422, Pages: 345-349, ISSN: 0009-2614
Basis set superposition error (BSSE) in density-functional calculations occurs when the extended Kohn-Sham orbitals are expanded in localised basis sets. but is absent when a plane-wave basis is used. Elimination of BSSE is essential for the accurate description of intermolecular forces. Linear-scaling methods are formulated in terms of local orbitals, making plane-waves an inappropriate choice of basis. In this work the BSSE in linear-scaling methods is studied in the context of hydrogen bonds. In particular it is shown that BSSE is eliminated by optimizing the local orbitals in situ using a systematic basis set equivalent to a set of plane-waves. (c) 2006 Elsevier B.V. All rights reserved.
Skylaris CK, Haynes PD, Mostofi AA, et al., 2006, Implementation of linear-scaling plane wave density functional theory on parallel computers, PHYS STATUS SOLIDI B, Vol: 243, Pages: 973-988
We describe the algorithms we have developed for linear-scaling plane wave density functional calculations on parallel computers as implemented in the ONETEP program. We outline how ONETEP achieves plane wave accuracy with a computational cost which increases only linearly with the number of atoms by optimising directly the single-particle density matrix expressed in a psinc basis set. We describe in detail the novel algorithms we have developed for computing with the psinc basis set the quantities needed in the evaluation and optimisation of the total energy within our approach. For our parallel computations we use the general Message Passing Interface (MPI) library of subroutines to exchange data between processors. Accordingly, we have developed efficient schemes for distributing data and computational load to processors in a balanced manner. We describe these schemes in detail and in relation to our algorithms for computations with a psinc basis. Results of tests on different materials show that ONETEP is an efficient parallel code that should be able to take advantage of a wide range of parallel computer architectures. (c) 2006 WILEYNCH Verlag GmbH & Co. KGaA, Weinheim.
Haynes PD, Mostofi AA, Skylaris CK, et al., 2006, ONETEP: linear-scaling density-functional theory with plane-waves, EMAG/NANO Conference on Imaging, Analysis and Fabrication on the Nanoscale, Publisher: IOP PUBLISHING LTD, Pages: 143-148, ISSN: 1742-6588
This paper provides a general overview of the methodology implemented in ONETEP (Order-N Electronic Total Energy Package), a parallel density-functional theory code for large-scale first-principles quantum-mechanical calculations. The distinctive features of ONETEP are linear-scaling in both computational effort and resources, obtained by making well-controlled approximations which enable simulations to be performed with plane-wave accuracy. Titanium dioxide clusters of increasing size designed to mimic surfaces are studied to demonstrate the accuracy and scaling of ONETEP.
Skylaris CK, Haynes PD, Mostofi AA, et al., 2005, Using ONETEP for accurate and efficient O(N) density functional calculations, JOURNAL OF PHYSICS-CONDENSED MATTER, Vol: 17, Pages: 5757-5769, ISSN: 0953-8984
Skylaris CK, Haynes PD, Mostofi AA, et al., 2005, Introducing ONETEP: Linear-scaling density functional simulations on parallel computers, JOURNAL OF CHEMICAL PHYSICS, Vol: 122, ISSN: 0021-9606
Mostofi AA, Haynes PD, Skylaris CK, et al., 2003, Preconditioned iterative minimization for linear-scaling electronic structure calculations, JOURNAL OF CHEMICAL PHYSICS, Vol: 119, Pages: 8842-8848, ISSN: 0021-9606
Mostofi AA, Skylaris CK, Haynes PD, et al., 2002, Total-energy calculations on a real space grid with localized functions and a plane-wave basis, COMPUTER PHYSICS COMMUNICATIONS, Vol: 147, Pages: 788-802, ISSN: 0010-4655
Skylaris CK, Mostofi AA, Haynes PD, et al., 2002, Nonorthogonal generalized Wannier function pseudopotential plane-wave method, PHYSICAL REVIEW B, Vol: 66, ISSN: 1098-0121
Skylaris CK, Mostofi AA, Haynes PD, et al., 2001, Accurate kinetic energy evaluation in electronic structure calculations with localized functions on real space grids, COMPUTER PHYSICS COMMUNICATIONS, Vol: 140, Pages: 315-322, ISSN: 0010-4655
Oliveira MJT, Papior N, Pouillon Y, et al., The CECAM Electronic Structure Library and the modular software development paradigm
First-principles electronic structure calculations are very widely usedthanks to the many successful software packages available. Their traditionalcoding paradigm is monolithic, i.e., regardless of how modular its internalstructure may be, the code is built independently from others, from thecompiler up, with the exception of linear-algebra and message-passinglibraries. This model has been quite successful for decades. The rapid progressin methodology, however, has resulted in an ever increasing complexity of thoseprograms, which implies a growing amount of replication in coding and in therecurrent re-engineering needed to adapt to evolving hardware architecture. TheElectronic Structure Library (\esl) was initiated by CECAM (European Centre forAtomic and Molecular Calculations) to catalyze a paradigm shift away from themonolithic model and promote modularization, with the ambition to extractcommon tasks from electronic structure programs and redesign them as free,open-source libraries. They include ``heavy-duty'' ones with a high degree ofparallelisation, and potential for adaptation to novel hardware within them,thereby separating the sophisticated computer science aspects of performanceoptimization and re-engineering from the computational science done byscientists when implementing new ideas. It is a community effort, undertaken bydevelopers of various successful codes, now facing the challenges arising inthe new model. This modular paradigm will improve overall coding efficiency andenable specialists (computer scientists or computational scientists) to usetheir skills more effectively. It will lead to a more sustainable and dynamicevolution of software as well as lower barriers to entry for new developers.
Goodwin ZAH, Vitale V, Liang X, et al., Hartree theory calculations of quasiparticle properties in twisted bilayer graphene
A detailed understanding of interacting electrons in twisted bilayer graphene(tBLG) near the magic angle is required to gain insights into the physicalorigin of the observed broken symmetry phases including correlated insulatorstates and superconductivity. Here, we present extensive atomistic Hartreetheory calculations of the electronic properties of tBLG in the (semi-)metallicphase as function of doping and twist angle. Specifically, we calculatequasiparticle properties, such as the band structure, density of states (DOS)and local density of states (LDOS), which are directly accessible inphotoemission and tunnelling spectroscopy experiments. We find thatquasiparticle properties change significantly upon doping - an effect which isnot captured by tight-binding theory. In particular, we observe that thepartially occupied bands flatten significantly which enhances the density ofstates at the Fermi level and explains the experimentally observed Fermi levelpinning. We predict a clear signature of this band flattening in the LDOS inthe AB/BA regions of tBLG which can be tested in scanning tunnelingexperiments. We also study the dependence of quasiparticle properties on thedielectric environment of tBLG and discover that these properties aresurprisingly robust as a consequence of the strong internal screening. Finally,we present a simple analytical expression for the Hartree potential whichenables the determination of quasiparticle properties without the need forself-consistent calculations.
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