# Jeongnim KimIntel · Data Platform Group

Jeongnim Kim

PhD

## About

111

Publications

13,494

Reads

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3,198

Citations

Citations since 2016

Additional affiliations

October 2012 - March 2014

October 2011 - March 2014

July 2001 - September 2011

## Publications

Publications (111)

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...

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...

\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...

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...