Jeongnim Kim

• PhD
• Principal Engineer at Intel

Data structures and algorithms are essential building blocks for programs, and \emph{distributed data structures}, which automatically partition data across multiple memory locales, are essential to writing high-level parallel programs. While many projects have designed and implemented C++ distributed data structures and algorithms, there has not b...

The Vienna Ab initio Simulation Package (VASP) is a widely used electronic structure code that originally exploits process‐level parallelism through the Message Passing Interface (MPI) for work distribution within and across nodes. Architectural changes of modern parallel processors urge programmers to address thread‐ and data‐level parallelism as...

QMCPACK is an open source quantum Monte Carlo package for ab-initio electronic structure calculations. It supports calculations of metallic and insulating solids, molecules, atoms, and some model Hamiltonians. Implemented real space quantum Monte Carlo algorithms include variational, diffusion, and reptation Monte Carlo. QMCPACK uses Slater-Jastrow...

QMCPACK is an open source quantum Monte Carlo package for ab-initio electronic structure calculations. It supports calculations of metallic and insulating solids, molecules, atoms, and some model Hamiltonians. Implemented real space quantum Monte Carlo algorithms include variational, diffusion, and reptation Monte Carlo. QMCPACK uses Slater-Jastrow...

We describe for the VASP application (a widely used electronic structure code written in FORTRAN) the transition from an MPI-only to a hybrid code base leveraging the three relevant levels of parallelism to be addressed when optimizing for an effective execution on modern computer platforms: multiprocessing, multithreading and SIMD vectorization. T...

The OpenMP (The OpenMP name is a registered trademark of the OpenMP Architecture Review Board.) application programming interface provides a simple way for programmers to write parallel programs that are portable between machines and vendors. Programmers parallelize their programs to obtain higher performance, but, as the number of cores per proces...

QMCPACK has enabled cutting-edge materials research on supercomputers for over a decade. It scales nearly ideally but has low single-node efficiency due to the physics-based abstractions using array-of-structures objects, causing inefficient vectorization. We present a systematic approach to transform QMCPACK to better exploit the new hardware feat...

QMCPACK has enabled cutting-edge materials research on supercomputers for over a decade. It scales nearly ideally but has low single-node efficiency due to the physics-based abstractions using array-of-structures objects, causing inefficient vectorization. We present a systematic approach to transform QMCPACK to better exploit the new hardware feat...

\alpha$-graphyne is a two-dimensional sheet of $sp$-$sp^2$ hybridized carbon atoms in a honeycomb lattice. While the geometrical structure is similar to that of graphene, the hybridized triple bonds give rise to electronic structure that is different from that of graphene. Similar to graphene, $\alpha$-graphyne can be stacked in bilayers with two s...

\alpha$-graphyne is a two-dimensional sheet of $sp$-$sp^2$ hybridized carbon atoms in a honeycomb lattice. While the geometrical structure is similar to that of graphene, the hybridized triple bonds give rise to electronic structure that is different from that of graphene. Similar to graphene, $\alpha$-graphyne can be stacked in bilayers with two s...

B-spline based orbital representations are widely used in Quantum Monte Carlo (QMC) simulations of solids, historically taking as much as 50% of the total run time. Random accesses to a large four-dimensional array make it challenging to efficiently utilize caches and wide vector units of modern CPUs. We present node-level optimizations of B-spline...

Quantum Monte Carlo (QMC) applications perform simulation with respect to an initial state of the quantum mechanical system, which is often captured by using a cubic B-spline basis. This representation is stored as a read-only table of coefficients and accesses to the table are generated at random as part of the Monte Carlo simulation. Current QMC...

Exploiting multilevel parallelism deserves consideration even if rejected in the past. OpenMP nesting is turned off by default by most implementations, and is generally consider unsafe by typical users due to concerns of oversubscription and the resulting poor application performance. Proper considerations of how to express and use nested paralleli...

We discuss several parallelization methods for multi-level hierarchical SMP systems using a stencil-based finite difference code. Performance comparisons and suggestions for OpenMP runtime improvements are provided.

We have applied the many-body ab initio diffusion quantum Monte Carlo (DMC) method to study Zn and ZnO crystals under pressure and the energetics of the oxygen vacancy, zinc interstitial, and hydrogen impurities in ZnO. We show that DMC is an accurate and practical method that can be used to characterize multiple properties of materials that are ch...

We have evaluated the successes and failures of the Hubbard-corrected density functional theory approach to study Mg doping of LiCoO2. We computed the effect of the U parameter on the energetic, geometric, and electronic properties of two possible doping mechanisms: (1) substitution of Mg onto a Co (or Li) site with an associated impurity state and...

van der Waals forces are notoriously difficult to account for from first principles. We have performed extensive calculations to assess the usefulness and validity of diffusion quantum Monte Carlo when predicting van der Waals forces. We present converged results for noble gas solids and clusters, archetypical van der Waals dominated systems, as we...

We have performed quantum Monte Carlo (QMC) simulations and density functional theory (DFT) calculations to study the equations of state of $MgSiO_3$ perovskite (Pv) and post-perovskite (PPv), up to the pressure and temperature conditions of the base of Earth's lower mantle. The ground state energies were derived using QMC and the temperature depen...

We develop an energy density matrix that parallels the one-body reduced density matrix (1RDM) for many-body quantum systems. Just as the density matrix gives access to the number density and occupation numbers, the energy density matrix yields the energy density and orbital occupation energies. The eigenvectors of the matrix provide a natural orbit...

We have applied the many-body $\textit{ab-initio}$ diffusion quantum monte carlo (DMC) method to calculate the band gap of ZnO and to study the oxygen vacancy in this material. DMC calculations clearly rule out the oxygen vacancy as the source of the persistent $\textit{n}$-type conductivity in ZnO. The DMC results were compared with Hartree-Fock,...

The accurate description of the thermodynamic and dynamical properties of liquid water from first-principles is a very important challenge to the theoretical community. This represents not only a critical test of the predictive capabilities of first-principles methods, but it will also shed light into the microscopic properties of such an important...

LiCoO 2 is widely used as cathode material for Li-ion batteries (1). However, the usable specific capacity of LiCoO 2 is limited to approximately half of its theoretical capacity. This limitation comes from the mechanical failure of the cathode when more than 50% of Li is deintercalated. The stability of LiCoO 2 and its electrochemical performance...

We have performed quantum Monte Carlo calculations to study the cohesion energetics of carbon allotropes, including sp(3)-bonded diamond, sp(2)-bonded graphene, sp-sp(2) hybridized graphynes, and sp-bonded carbyne. The computed cohesive energies of diamond and graphene are found to be in excellent agreement with the corresponding values determined...

In view of the continuous theoretical efforts aimed at an accurate microscopic description of the strongly correlated transition metal oxides and related materials, we show that with continuum quantum Monte Carlo (QMC) calculations it is possible to obtain the value of the spin superexchange coupling constant of a copper oxide in a quantitatively e...

A statistical method is derived for the calculation of thermodynamic properties of many-body systems at low temperatures. This method is based on the self-healing diffusion Monte Carlo method for complex functions [F. A. Reboredo, J. Chem. Phys. 136, 204101 (2012)] and some ideas of the correlation function Monte Carlo approach [D. M. Ceperley and...

Traditional petascale applications, such as QMCPack, can scale their computations to completely utilize modern supercomputers like Titan, but they cannot scale their I/O. To preserve scalability, scientists cannot save data at the granularity needed to enable scientific discovery and are forced to use large intervals between two checkpoint calls. I...

Solid atomic hydrogen is one of the simplest systems to undergo a metal-insulator transition. Near the transition, the electronic degrees of freedom become strongly correlated and their description provides a difficult challenge for theoretical methods. As a result, the order and density of the phase transition are still subject to debate. In this...

We establish a physically meaningful representation of a quantum energy density for use in Quantum Monte Carlo calculations. The energy density operator, defined in terms of Hamiltonian components and density operators, returns the correct Hamiltonian when integrated over a volume containing a cluster of particles. This property is demonstrated for...

Benchmark diffusion quantum monte-carlo (DMC) studies of the adsorption and diffusion of atomic lithium in graphite are compared with density functional theory (DFT) calculations using several van der Waals methods. The charge transfer is captured adequately with conventional local density functionals. At fixed geometries, these yield surprisingly...

Defect formation energies require expensive energy difference calculations between defective and bulk systems over a range of system sizes. At the point of convergence, subregions added to represent larger systems no longer contribute to the formation energy and therefore display similar local energetics. A recent formulation of the energy density...

With advances in algorithms and growing computing powers, quantum Monte Carlo (QMC) methods have become a leading contender for high accuracy calculations for the electronic structure of realistic systems. The performance gain on recent HPC systems is largely driven by increasing parallelism: the number of compute cores of a SMP and the number of S...

Quantum Monte Carlo (QMC) methods have received considerable attention over the last decades due to their great promise for providing a direct solution to the many-body Schrodinger equation in electronic systems. Thanks to their low scaling with number of particles, QMC methods present a compelling competitive alternative for the accurate study of...

Van der Waals forces are as ubiquitous as infamous. While post-Hartree-Fock methods enable accurate estimates of these forces in molecules and clusters, they remain elusive for dealing with many-electron condensed phase systems. We present Quantum Monte Carlo [1,2] results for condensed van der Waals systems. Interatomic many-body contributions to...

Continuum quantum Monte Carlo (QMC) methods are a leading contender for high accuracy calculations for the electronic structure of realistic systems, especially on massively parallel high-performance computers (HPC). The performance gain on recent HPC systems is largely driven by increasing parallelism: the number of compute cores of a SMP and the...

We establish a physically meaningful representation of a quantum energy density for use in quantum Monte Carlo calculations. The energy density operator, defined in terms of Hamiltonian components and density operators, returns the correct Hamiltonian when integrated over a volume containing a cluster of particles. This property is demonstrated for...

At low density BCC hydrogen undergoes a metal-insulator transition. We compute the zero temperature equation of state for the paramagnetic and anti-ferromagnetic phases using diffusion Quantum Monte Carlo. We predict the phase transition density, investigate the shape of the anti-ferromagnetic curve, and compare to previous results

More accurate than mean-field methods and more scalable than quantum chemical methods, continuum quantum Monte Carlo (QMC) is an invaluable tool for predicting the properties of matter from fundamental principles. Because QMC algorithms offer multiple forms of parallelism, they're ideal candidates for acceleration in the many-core paradigm.

Quantum Monte Carlo (QMC) methods such as variational Monte Carlo and fixed node diffusion Monte Carlo depend heavily on the quality of the trial wave function. Although Slater-Jastrow wave functions are the most commonly used variational ansatz in electronic structure, more sophisticated wave functions are critical to ascertaining new physics. One...

Wave function optimization is essential for both the accuracy and efficiency of diffusion, reptation, and variational quantum Monte Carlo (QMC). In this talk we outline the wave function optimization strategy used in the QMC software package QMCPACK developed at the University of Illinois. We use an extension of the linear optimization method origi...

Quantum Monte Carlo (QMC) methods such as variational and diffusion Monte Carlo depend heavily on the quality of the trial wave function. Although Slater-Jastrow wave functions are the most commonly used variational ansatz, more sophisticated wave functions are critical to ascertaining new physics. One such wave function is the multislater- Jastrow...

We present a quantum Monte Carlo study of many molecular structures of Ti-ethylene with up to 5 H2 molecules. These structures have been of recent interest due to energetics favorable for reversibly storing hydrogen.ootnotetextE. Durgun et al., Phys. Rev. Lett. 97, 226102 (2006). Diffusion Monte Carlo is employed with the fixed node approximation a...

The simulation of defect dynamics and evolution is a technologically relevant challenge for computational materials science. The diffusion of small defects in silicon unfolds as a sequence of structural transitions. The relative infrequency of transition events requires simulation over extremely long time scales. We simulate the diffusion of small...

Quantum Monte Carlo (QMC) simulations are among the most accurate ab initio methods describing the many-particle systems properties. Computations perform accesses to an ensemble data structure describing the system quantum state. The ensemble data grows rapidly with the system size and desired resolution. For leadership-scale simulations, this stru...

We study the electronic structure of a spherical jellium in the presence of a central Gaussian impurity. We test how well the resulting inhomogeneity effects beyond spherical jellium are reproduced by several approximations of density functional theory (DFT). Four rungs of Perdew's ladder of DFT functionals, namely local density approximation (LDA)...

Quantum Monte Carlo (QMC) methods are often used to calculate properties of many body quantum systems. The main cost of many QMC methods, for example the variational Monte Carlo (VMC) method, is in constructing a sequence of Slater matrices and computing the ratios of determinants for successive Slater matrices. Recent work has improved the scaling...

We present experimental and theoretical results on the momentum distribution and the quasiparticle renormalization factor in sodium. From an x-ray Compton-profile measurement of the valence-electron momentum density, we derive its discontinuity at the Fermi wavevector. This yields an accurate measure of the renormalization factor that we compare wi...

We develop an all-electron quantum Monte Carlo (QMC) method for solids that does not rely on pseudopotentials, and use it to construct a primary ultra-high-pressure calibration based on the equation of state of cubic boron nitride. We compute the static contribution to the free energy with the QMC method and obtain the phonon contribution from dens...

We study the effects of a model impurity in a spherical jellium with quantum Monte Carlo (QMC) methods. The closed-shell energies and densities of jellium spheres have been studied previously using density functional theory (DFT) as well as QMC methods [1,2]. In this study, we begin by reproducing the previous results. Second, we add an impurity to...

Silicon Carbide (SiC) is an important semiconductor used in high temperature electronic applications because of its large excitation energy. We use diffusion quantum Monte Carlo (DMC) to calculate some of it's electronic, physical, and optical properties. An analysis of the symmetry of the trial wave-function's single particle orbitals is required...

We present a quantum Monte Carlo study of molecular TiH2 and Ti-ethylene-hydrogen complexes which have been of recent interest for their relation to systems that can reversibly adsorb hydrogen.footnotetextE. Durgun et al., Phys. Rev. Lett. 97, 226102 (2006).^,footnotetextJ. A. Platts, J. Mol. Struct. 545, 111 (2001).^,footnotetextB. Ma, C. L. Colli...

Continuum quantum Monte Carlo (QMC) has proved to be an invaluable tool for predicting the properties of matter from fundamental principles. The multiple forms of parallelism afforded by QMC algorithms make it an ideal candidate for acceleration in the many-core paradigm on graphical processing units (GPUs). We present the results of porting the QM...

LDA+U and LDA+DMFT are successful methods for determining the electronic structure of FeO under pressure, but they suffer from two deficiencies. The extreme sensitivity of the spin collapse in MnO on the parameters U and J casts doubt upon the predictive power of the methods^1. Additionally, the symmetry of the occupation matrix has a profound effe...

With advances in algorithms and the changing landscape of high performance computers (HPC), the quantum Monte Carlo method has become a leading contender for high accuracy calculations for the electronic structure of realistic systems. QMC, being statistical, is naturally scalable to a large number of processors. We discuss QMC implementations to o...

FeO has a rich behavior under pressure, exhibiting a structural phase transition as well as an insulator-metal transition and a spin collapse. The electronic transitions have been particularly difficult to explain because of the failure of Density Functional Theory (DFT) to capture the electronic state of FeO. We present results from three differen...

We have developed a fundamental high-temperature and high-pressure scale based on cubic boron nitride (cBN) using a combination of Quantum Monte Carlo (QMC) for the static contribution along with density functional perturbation theory (DFPT) for the thermal pressure. The anharmonic Raman frequency was determined as a function of pressure by solving...

We present a quantum Monte Carlo study of the hydrogen-benzene system where binding is very weak. We demonstrate that the binding is well described at both variational Monte Carlo (VMC) and diffusion Monte Carlo (DMC) levels by a Jastrow correlated single determinant geminal wave function with an optimized compact basis set that includes diffuse or...

Over the past two decades, continuum quantum Monte Carlo (QMC) has proved to be an invaluable tool for predicting of the properties of matter from fundamental principles. By solving the Schrödinger equation through a stochastic projection, it achieves the greatest accuracy and reliability of methods available for physical systems containing more th...

We present a quantum Monte Carlo study of the hydrogen-benzene system where binding is very weak. We demonstrate that the binding is well described at both variational Monte Carlo (VMC) and diffusion Monte Carlo (DMC) levels by a Jastrow correlated single determinant geminal wave function with an optimized compact basis set that includes diffuse or...

Assaraf and Caffarel have developed a systematic method for deriving reduced variance estimators for observables for quantum Monte Carlo calculations and have applied to forces[1], the one body density and the spherical and system averaged pair density. It has yet to be applied to a thermodynamic observable. In this work we derive an expression for...

We present ab initio calculations of the crystalline phases of C28: hyperdiamond and hyperlonsdaleite, in their pristine and endohedrally doped forms. These are hard materials with strong covalent bonds between the C28 molecules, and yet their electronic properties have remarkable similarities to the weakly bonded C28H4 molecular solids previously...

We report the results of a study of the ground-state and excitation energies of Ge atoms, molecules, and clusters as large as Ge29H36, using quantum Monte Carlo (QMC) for the valence electrons. QMC is one of the most accurate many-body methods; however, its accuracy is limited by the way the core is treated, which is especially important for atoms...

Many prospective hydrogen storage systems contain carbon scaffolding comprised of benzene-like structural units. The binding energy of H2 with these benzene-like rings is below the ˜ 0.2-0.4 eV/H2 target necessary for reversible adsorption^1. Here we study the hydrogen-benzene system using quantum Monte Carlo (MC) methods suitable for resolving sma...

Computation of accurate energy differences is of primary importance in the study of transformations as those occurring in solid to solid phase transitions or chemical reactions. In stochastic quantum simulations this can be done efficiently, employing correlated sampling techniques whereby fluctuations cancel with each other leading to results with...

Quantum Monte Carlo (QMC) calculations are presented for energies of ground and excited states of Ge atom and hydrogen passivated closed-shell molecules and clusters: GeH4, Ge2H6, Ge5H12, Ge10H16 and Ge29H36. We compare the results for excitations with previous QMC and time-dependant Density Functional Theory (TD- DFT) done for the corresponding Si...

The morphology of nanoscopic Ag grains significantly affects the phonons. Atomistic simulations show that realistic nanograin models display complex vibrational properties. (1) Single-crystalline grains. Nearly-pure torsional and radial phonons appear at low frequencies. For low-energy, faceted models, the breathing mode and acoustic gap (lowest fr...

Quantum Monte Carlo (QMC) calculations of the optical gaps of hydrogen-passivated Ge clusters of size 1-3 nm are presented. Although QMC methods are the most accurate methods known for interacting electrons, there are outstanding challenges in applications to materials containing heavy atoms such as Ge. The replacement of core- electrons by a pseud...

Extended 311 defects and interstitial clusters in silicon are commonly believed to play important roles in dopant transient enhanced diffusion. In experiments, DLTS signals correspond to both localized and extended defects are observed in the silicon band gap. We perform GWA calculation for the quasi-particle levels of some of the important defects...

We present {\it ab initio} density-functional calculations of molecular solids formed from C$_{28}$-derived closed-shell fullerenes. Solid C$_{28}$H$_4$ is found to bind weakly and exhibits many of the electronic structure features of solid C$_{60}$ with an enhanced electron-phonon interaction potential. We show that chemical doping of this structu...

The combination of long-time, tight-binding molecular dynamics and real-time multiresolution analysis techniques reveals the complexity of small silicon interstitial defects. The stability of identified structures is confirmed by ab initio relaxations. The majority of structures were previously unknown, demonstrating the effectiveness of the approa...

Although quantum Monte Carlo (QMC) methods are the most accurate methods known for many interacting electrons, there are severe challenges in applications to materials containing heavy atoms. The outstanding issue is how to treat the effects of the core electrons: all-electron calculations are not feasible because of the large number of electrons a...

The principle of the nearsightness of the single-particle density matrix [1] is the ground for so-called O(N) method whose computations scale linearly with respect to the system size N. Variational O(N) methods using generalized Wannier functions or density matrices [2] relies on the minimum energy principle: E_tot(N) = Tr(rho H) with the single-pa...

Motivated by theoretical models which predict that caged fullerenes with higher curvature should have larger electron-phonon coupling [1] and potentially higher superconducting transition temperature T_c, we explore possible molecular solids constructed from C_28-derived cages. The partially filled molecular states of C_28 lead to the formation of...

Many-body levels of optically excited and multiply charged InAs nanocrystals are studied with the semiempirical tight-binding model. Single-particle levels of unstrained spherical InAs nanocrystals are described by the sp3d5s* nearest-neighbor tight-binding model including spin-orbit coupling. For optically excited InAs nanocrystals, first-order co...

The simulation of defect dynamics and evolution is a technologically relevant challenge for computational materials science. The diffusion of small defects in silicon unfolds as a sequence of structural transitions. The relative infrequency of transition events requires simulation over extremely long time scales. We simulate the diffusion of small...

Computational materials science increasingly assumes a greater responsibility in developing materials for high-performance applications. This will be possible only if new theoretical ideas can be quickly realized with effective algorithms that port effortlessly to new platforms. That is the goal of OHMMS: a Object-oriented High-performance Multi-sc...

We investigate the structural properties and electronic structure of rodlike planar 311 defects in silicon using a multiscale approach. We first determine possible defect geometries by tight-binding molecular dynamics simulations. These are subsequently refined by ab-initio local density approximation (LDA) relaxations. The resulting equilibrium st...

We simulate the diffusion of silicon single-, di-, and tri-interstitial defects (I_1, I_2, I_3) with an accurate tight-binding (TB) potential. The OHMMS simulation code generates a total of 0.25 mus of simulation time at temperatures ranging from 800-1100 K. OHMMS detects transitions and metastable structures with a wavelet-based, real-time multire...

The simulation of defect dynamics and evolution is a technologically relevant challenge for computational materials science. The diffusion of small defects in silicon unfolds as a sequence of structural transitions. The relative infrequency of transition events requires simulation over extremely long time scales. We simulate the diffusion of small...

We apply parallel replica dynamics to simulate the growth of silicon interstitial clusters. Existing interstitial clusters are efficient traps for mobile interstitial and di-interstitial defects. For clusters involving more than four interstitials, many metastable structures are achieved by local bonding rearrangements. The shape of interstitial nu...

We discuss the energetics of small Si-interstitial clusters containing n = 4-8 interstitials. The structural and dynamical properties of Si-interstitial defects are key to understanding of dopant dynamics. Small interstitial clusters can provide the growth nuclei for extended defects and can release mobile interstitials leading to enhanced diffusio...

The simulation of defect dynamics is a technologically relevant challenge for computational materials science. The dynamics of defect structures unfolds as a sequence of thermally induced transitions. Identifying and characterizing reaction paths, as well as extracting dynamical quantities, is important for modeling the macroscopic properties of re...

The simulation of defect dynamics (e.g., transient enhanced diffusion of boron in the presence of silicon interstitials) is a technologically relevant challenge for computational materials science. The dynamics of defect structures in bulk unfolds as a sequence of thermally induced structural transitions. Identifying and characterizing reaction pat...

Structural and dynamical properties of silicon interstitial defects are extracted from extensive atomistic simulations using ab initio total energy calculations. With increasing number of interstitials, the stable defect shape evolves from compact to chain-like to rod-like. The rodlike {l_brace}311{r_brace} defect, formed from (011) interstitial ch...

We fit an empirical potential for silicon using the modified embedded atom (MEAM) functional form, which contains a nonlinear function of a sum of pairwise and three-body terms. The three-body term is similar to the Stillinger-Weber form. We parametrized our model using five cubic splines, each with 10 fitting parameters, and fitted the parameters...

We propose(Kim et al.), Phys. Rev. Lett. 83, 1990 (1999). a di-interstitial model for the P6 center commonly observed in ion-implanted silicon. The model, especially the transition paths between different defect orientations, can explain the thermally activated transition of the P6 center from low-temperature C_1h symmetry to room-temperature D_2d...

Extended Si interstitial defects, induced by boron-ion-implantation, are believed to provide Si interstitials responsibile for the transient-enhanced-diffusion of boron during annealing processes. Experiments and simulations have suggested that extended-interstitial-defect growth occurs via Ostwald ripening process. The size-dependence of the energ...

Trends in the growth of extended interstitial defects are extracted from extensive tight-binding and ab inito local density approximation simulations. With an increasing number of interstitials, the stable defect shape evolves from compact to chainlike to rodlike. The rodlike 311 defect, formed from (011) interstitial chains, is stabilized as it gr...

We have carried out a series of atomistic simulations of the room temperature bonding of clean, defect-free Si wafers under UHV conditions using Classical Molecular Dynamics (CMD) and Self-Consistent Tight-Binding (SCTB) Our simulations indicate that even when the wafers are perfectly aligned, bonding does not typically result in the formation of b...

We propose a di-interstitial model for the P6 center commonly observed in ion implanted silicon. The di-interstitial structure and transition paths between different defect orientations can explain the thermally activated transition of the P6 center from low-temperature C1h to room-temperature D2d symmetry. The activation energy for the defect reor...

We calculate the electronic structures of pyramidal quantum dots with supercells containing 250 000 atoms, using spin-orbit-coupled, nonlocal, empirical pseudopotentials. We compare the results with previous theoretical calculations. Our calculation circumvents the approximations underlying the conventional effective-mass approach: we describe the...

For the latest EPM potentials, please see appendix A in Physical Review B, 59, 15270 (1999)

Using a pseudopotential plane-wave approach, we have calculated the electronic structure of strained InAs pyramidal quantum dots embedded in a GaAs matrix, for a few height (h)-to-base(b) ratios, corresponding to different facet orientations \{101\}, \{113\}, and \{105\}. We find that the dot shape (not just size) has a significant effect on its el...

We have developed a linear scaling method (O(N)) for electronic structure calculations and molecular dynamics simulations based on an orbital formulation.(F. Mauri and G. Galli, Phys. Rev. B 50), 4316 (1994); J. Kim et al. ibid. 52, 1640 (1995). The linear increase of the computational time with respect to the system size is achieved by using local...

We study the energetics of a Si 113 reconstructed surface with respect to the interstitial concentration and perform dynamics simulations at high temperatures in a tight-binding representation. Recent experiments and first-principle calculations(J. Dabrowski et al.), Phys. Rev. Lett. 73, 1660 (1994). have shown that Si 113 surface is very stable du...

Using tight-binding molecular dynamics, we have performed computer experiments to mimic the gas phase growth of a disordered solid composed of C28 fullerenes. The growth has been simulated by repeated low energy collisions of molecules coming from random directions. The resulting solid is composed of undamaged C28 cages, with most fullerenes being...