arXiv subjects

Richard G. Hennig

Publications and source records attributed to Richard G. Hennig.

43 records · Page 3Linked to original sources

arXiv2013

Implicit solvation model for density-functional study of nanocrystal surfaces and reaction pathways

Solid-liquid interfaces are at the heart of many modern-day technologies and provide a challenge to many materials simulation methods. A realistic first-principles computational study of such systems entails the inclusion of solvent effects. In this work we implement an implicit solvation model that has a firm theoretical foundation into the widely used density-functional code VASP. The implicit solvation model follows the framework of joint density functional theory. We describe the framework, our algorithm and implementation, and benchmarks for small molecular systems. We apply the solvation model to study the surface energies of different facets of semiconducting and metallic nanocrystals and the S$_{\text{N}} 2$ reaction pathway. We find that solvation reduces the surface energies of the nanocrystals, especially for the semiconducting ones and increases the energy barrier of the S$_{\text{N}} 2$ reaction.

cond-mat.mtrl-sci↗

arXiv2012

Ab Initio Discovery of Novel Crystal Structure Stability in Barium and Sodium-Calcium Compounds under Pressure using DFT

Group I/II materials exhibit unexpected structural phase transitions at high pressures, providing potential insight into the origins of elemental superconductivity. We present here a computational study of elemental barium and binary sodium-calcium alloys to identify both known and unknown phases of barium under pressure, as well as stable high-pressure compounds in the immiscible Na-Ca system. To predict stability, we performed density functional theory calculations on randomly generated structures and evolved them using a genetic algorithm. For barium, we observed all of the expected phases and a number of new metastable structures, excluding the incommensurate Ba-IV structure. We also observed a heretofore unreported structure (α-Sm) predicted to be the ground state from 30-42 GPa. In the Na-Ca system, we demonstrate feasibility of our search method, but have been unable to predict any stable compounds. These results have improved the efficacy of the genetic algorithm, and should provide many promising directions for future work.

cond-mat.mtrl-sci↗

arXiv2012

Framework for solvation in quantum Monte Carlo

Employing a classical density-functional description of liquid environments, we introduce a rigorous method for the diffusion quantum Monte Carlo calculation of free energies and thermodynamic averages of solvated systems that requires neither thermodynamic sampling nor explicit solvent electrons. We find that this method yields promising results and small convergence errors for a set of test molecules. It is implemented readily and is applicable to a range of challenges in condensed matter, including the study of transition states of molecular and surface reactions in liquid environments.

physics.comp-ph↗

arXiv2012

Importance of High Angular-Momentum Channels in Pseudopotentials for Quantum Monte Carlo

Quantum Monte Carlo methods provide in principle an accurate treatment of the many-body problem of the ground and excited states of condensed systems. In practice, however, uncontrolled errors such as those arising from the fixed-node and pseudopotential approximations often limit the quality of results. We show that the accuracy of quantum Monte Carlo calculations is limited by using available pseudopotentials. In particular, it is necessary to include angular momentum channels in the pseudopotential for excited angular momentum states and to choose the local channel appropriately to obtain accurate results. Variational and diffusion Monte Carlo calculations for Zn, O, and Si atoms and ions demonstrate that these issues can affect total energies by up to several eV for common pseudopotentials. Adding higher-angular momentum channels into the pseudopotential description reduces such errors drastically without a significant increase in computational cost.

cond-mat.mtrl-sci↗

arXiv2010

Accuracy of Quantum Monte Carlo Methods for Point Defects in Solids

Quantum Monte Carlo approaches such as the diffusion Monte Carlo (DMC) method are among the most accurate many-body methods for extended systems. Their scaling makes them well suited for defect calculations in solids. We review the various approximations needed for DMC calculations of solids and the results of previous DMC calculations for point defects in solids. Finally, we present estimates of how approximations affect the accuracy of calculations for self-interstitial formation energies in silicon and predict DMC values of 4.4(1), 5.1(1) and 4.7(1) eV for the X, T and H interstitial defects, respectively, in a 16(+1)-atom supercell.

cond-mat.mtrl-sci↗

arXiv2009

The energy landscape of silicon systems and its description by force fields, tight binding schemes, density functional methods and Quantum Monte Carlo methods

The accuracy of the energy landscape of silicon systems obtained from various density functional methods, a tight binding scheme and force fields is studied. Quantum Monte Carlo results serve as quasi exact reference values. In addition to the well known accuracy of DFT methods for geometric ground states and metastable configurations we find that DFT methods give a similar accuracy for transition states and thus a good overall description of the energy landscape. On the other hand, force fields give a very poor description of the landscape that are in most cases too rugged and contain many fake local minima and saddle points or ones that have the wrong height.

physics.comp-ph↗

arXiv2005

Fast Diffusion Mechanism of Silicon Tri-interstitial Defects

We reveal the microscopic self-diffusion process of compact tri-interstitials in silicon using a combination of molecular dynamics and nudged elastic band methods. We find that the compact tri-interstitial moves by a collective displacement, involving both translation and rotation, of five atoms in a screw-like motion along $[111]$ directions. The elucidation of this pathway demonstrates the utility of combining tight-binding molecular dynamics with \textit{ab initio} density functional calculations to probe diffusion mechanisms. Using density functional theory to obtain diffusion barriers and the prefactor, we calculate a diffusion constant of $ 4 \cdot 10^{-5} \exp (- 0.49 {\rm eV} / k_{B} T) {\rm cm^2/s} $. Because of the low diffusion barrier, $I_{3}^{b}$ diffusion may be an important process under conditions such as ion implantation that creates excess interstitials, hence favoring formation of interstitial clusters.

cond-mat.mtrl-sci↗