Koblar A. Jackson

Bio: Koblar A. Jackson is an academic researcher from Central Michigan University. The author has contributed to research in topics: Density functional theory & Cluster (physics). The author has an hindex of 37, co-authored 126 publications receiving 22881 citations. Previous affiliations of Koblar A. Jackson include George Mason University & University of Wisconsin-Madison.

Atoms, molecules, solids, and surfaces: Applications of the generalized gradient approximation for exchange and correlation.

Abstract: Generalized gradient approximations (GGA's) seek to improve upon the accuracy of the local-spin-density (LSD) approximation in electronic-structure calculations. Perdew and Wang have developed a GGA based on real-space cutoff of the spurious long-range components of the second-order gradient expansion for the exchange-correlation hole. We have found that this density functional performs well in numerical tests for a variety of systems: (1) Total energies of 30 atoms are highly accurate. (2) Ionization energies and electron affinities are improved in a statistical sense, although significant interconfigurational and interterm errors remain. (3) Accurate atomization energies are found for seven hydrocarbon molecules, with a rms error per bond of 0.1 eV, compared with 0.7 eV for the LSD approximation and 2.4 eV for the Hartree-Fock approximation. (4) For atoms and molecules, there is a cancellation of error between density functionals for exchange and correlation, which is most striking whenever the Hartree-Fock result is furthest from experiment. (5) The surprising LSD underestimation of the lattice constants of Li and Na by 3--4 % is corrected, and the magnetic ground state of solid Fe is restored. (6) The work function, surface energy (neglecting the long-range contribution), and curvature energy of a metallic surface are all slightly reduced in comparison with LSD. Taking account of the positive long-range contribution, we find surface and curvature energies in good agreement with experimental or exact values. Finally, a way is found to visualize and understand the nonlocality of exchange and correlation, its origins, and its physical effects.

Variational mesh for quantum-mechanical simulations.

Abstract: Developpement d'un algorithme variationnel pour generer une maille d'integration a utiliser dans les calculs de structures electroniques. La methode inclut un nouvel algorithme pour integrer au-dela des regions interstitielles arbitraires, et une technique d'integration des spheres atomiques amelioree (les erreurs diminuant avec le nombre de points radiaux). Les points de la maille et les elements de volume sont optimises afin de s'assurer qu'un grand nombre d'integrales sont evaluees exactement. Explicitation des deux idees retenues pour optimiser la maille. On montre comment les algorithmes de force peuvent etre utilises pour affiner les mailles resultantes. La precision et l'efficacite de cette methode sont evaluees en presentant des integrations de densite de charge sur une variete de clusters isoles. Comparaison de cette methode aux algorithmes conventionnels a points speciaux. Exemple de l'etat fondamental tetraedrique du fragment cristallin C 5 H 12

Accurate forces in a local-orbital approach to the local-density approximation.

Abstract: Les forces atomiques sont calculees comme des derivees propres de l'energie totale, incluant les necessaires corrections de l'ensemble de base Le potentiel de l'approximation est evalue exactement sur une maille d'integration determinee de facon variationnelle La methode est appliquee au dimere de molybdene, a la molecule d'ethylene et au monomere NaCl

Raman-active modes of a − GeSe 2 and a − GeS 2 : A first-principles study

Abstract: We have used a recently developed computational technique based on density-functional theory to study the Raman-active modes of amorphous ${\mathrm{GeSe}}_{2}$ and ${\mathrm{GeS}}_{2}.$ Vibrational modes and the associated Raman activities for three cluster building blocks of the glasses are calculated directly from first principles. The positions of the calculated symmetric-stretch modes in the cluster models are in excellent agreement with sharp features in the observed spectra. Moreover, simulated spectra based on the cluster results are in good agreement with experiment, accounting for all the observed features in the bond-stretch region of the spectra. The cluster results suggest a new interpretation for the 250 ${\mathrm{cm}}^{\ensuremath{-}1}$ mode appearing in the spectra of Ge-rich samples in the ${\mathrm{Ge}}_{x}{\mathrm{S}}_{1\ensuremath{-}x}$ family.

Density‐functional thermochemistry. III. The role of exact exchange

Abstract: Despite the remarkable thermochemical accuracy of Kohn–Sham density‐functional theories with gradient corrections for exchange‐correlation [see, for example, A. D. Becke, J. Chem. Phys. 96, 2155 (1992)], we believe that further improvements are unlikely unless exact‐exchange information is considered. Arguments to support this view are presented, and a semiempirical exchange‐correlation functional containing local‐spin‐density, gradient, and exact‐exchange terms is tested on 56 atomization energies, 42 ionization potentials, 8 proton affinities, and 10 total atomic energies of first‐ and second‐row systems. This functional performs significantly better than previous functionals with gradient corrections only, and fits experimental atomization energies with an impressively small average absolute deviation of 2.4 kcal/mol.

Fast parallel algorithms for short-range molecular dynamics

Abstract: Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

QUANTUM ESPRESSO: a modular and open-source software project for quantum simulations of materials

Abstract: QUANTUM ESPRESSO is an integrated suite of computer codes for electronic-structure calculations and materials modeling, based on density-functional theory, plane waves, and pseudopotentials (norm-conserving, ultrasoft, and projector-augmented wave). The acronym ESPRESSO stands for opEn Source Package for Research in Electronic Structure, Simulation, and Optimization. It is freely available to researchers around the world under the terms of the GNU General Public License. QUANTUM ESPRESSO builds upon newly-restructured electronic-structure codes that have been developed and tested by some of the original authors of novel electronic-structure algorithms and applied in the last twenty years by some of the leading materials modeling groups worldwide. Innovation and efficiency are still its main focus, with special attention paid to massively parallel architectures, and a great effort being devoted to user friendliness. QUANTUM ESPRESSO is evolving towards a distribution of independent and interoperable codes in the spirit of an open-source project, where researchers active in the field of electronic-structure calculations are encouraged to participate in the project by contributing their own codes or by implementing their own ideas into existing codes.

First-principles simulation: ideas, illustrations and the CASTEP code

Abstract: First-principles simulation, meaning density-functional theory calculations with plane waves and pseudopotentials, has become a prized technique in condensed-matter theory. Here I look at the basics of the suject, give a brief review of the theory, examining the strengths and weaknesses of its implementation, and illustrating some of the ways simulators approach problems through a small case study. I also discuss why and how modern software design methods have been used in writing a completely new modular version of the CASTEP code.