arXiv subjects

Laura E. Ratcliff

Publications and source records attributed to Laura E. Ratcliff.

29 records · Page 2Linked to original sources

arXiv2018

Zirconia and hafnia polymorphs -- ground state structural properties from diffusion Monte Carlo

Zirconia (zirconium dioxide) and hafnia (hafnium dioxide) are binary oxides used in a range of applications. Because zirconium and hafnium are chemically equivalent, they have three similar polymorphs, and it is important to understand the properties and energetics of these polymorphs. However, while density functional theory calculations can get the correct energetic ordering, the energy differences between polymorphs depend very much on the specific density functional theory approach, as do other quantities such as lattice constants and bulk modulus. We have used highly accurate quantum Monte Carlo simulations to model the three zirconia and hafnia polymorphs. We compare our results for structural parameters, bulk modulus, and cohesive energy with results obtained from density functional theory calculations. We also discuss comparisons of our results with existing experimental data, in particular for structural parameters where extrapolation to zero temperature can be attempted. We hope our results of structural parameters as well as for cohesive energy and bulk modulus can serve as benchmarks for density-functional theory based calculations and as a guidance for future experiments.

cond-mat.mtrl-sci↗

arXiv2017

Accurate, Large-Scale and Affordable Hybrid-PBE0 Calculations with GPU-Accelerated Supercomputers

Performing high accuracy hybrid functional calculations for condensed matter systems containing a large number of atoms is at present computationally very demanding - when not out of reach - if high quality basis sets are used. We present a highly efficient multiple GPU implementation of the exact exchange operator which allows hybrid functional density-functional theory calculations with systematic basis sets without additional approximations for up to a thousand atoms. This method is implemented in a portable real-space-based algorithm, released as an open-source package. With such a framework hybrid DFT calculations of high quality become accessible on state-of-the-art supercomputers within a time-to-solution of the same order of magnitude as traditional semilocal-GGA functionals.

cond-mat.mtrl-sci↗

arXiv2017

Complexity Reduction in Large Quantum Systems: Reliable Electrostatic Embedding for Multiscale Approaches via Optimized Minimal Basis Functions

Given a partition of a large system into an active quantum mechanical (QM) region and its environment, we present a simple way of embedding the QM region into an effective electrostatic potential representing the environment. This potential is generated by partitioning the environment into well defined fragments, and assigning each one a set of electrostatic multipoles, which can then be used to build up the electrostatic potential. We show that, providing the fragments and the projection scheme for the multipoles are chosen properly, this leads to an effective electrostatic embedding of the active QM region which is of equal quality as a full QM calculation. We coupled our formalism to the DFT code BigDFT, which uses a minimal set of localized in-situ optimized basis functions; this property eases the fragment definition while still describing the electronic structure with great precision. Thanks to the linear scaling capabilities of BigDFT, we can compare the modeling of the electrostatic embedding with results coming from unbiased full QM calculations of the entire system. This enables a reliable and controllable setup of an effective coarse-graining approach, coupling together different levels of description, which yields a considerable reduction in the degrees of freedom and thus paves the way towards efficient QM/QM and QM/MM methods for the treatment of very large systems.

physics.chem-ph↗

arXiv2017

Complexity Reduction in Large Quantum Systems: Fragment Identification and Population Analysis via a Local Optimized Minimal Basis

We present, within Kohn-Sham Density Functional Theory calculations, a quantitative method to identify and assess the partitioning of a large quantum mechanical system into fragments. We then show how within this framework simple generalizations of other well-known population analyses can be used to extract, from first principles, reliable electrostatic multipoles for the identified fragments. Our approach reduces arbitrariness in the fragmentation procedure, and enables the possibility to assess, quantitatively, whether the corresponding fragment multipoles can be interpreted as observable quantities associated to a system's moiety. By applying our formalism within the code BigDFT, we show that the use of a minimal set of in-situ optimized basis functions allows at the same time a proper fragment definition and an accurate description of the electronic structure.

physics.chem-ph↗

arXiv2016

Challenges in Large Scale Quantum Mechanical Calculations

During the past decades, quantum mechanical methods have undergone an amazing transition from pioneering investigations of experts into a wide range of practical applications, made by a vast community of researchers. First principles calculations of systems containing up to a few hundred atoms have become a standard in many branches of science. The sizes of the systems which can be simulated have increased even further during recent years, and quantum-mechanical calculations of systems up to many thousands of atoms are nowadays possible. This opens up new appealing possibilities, in particular for interdisciplinary work, bridging together communities of different needs and sensibilities. In this review we will present the current status of this topic, and will also give an outlook on the vast multitude of applications, challenges and opportunities stimulated by electronic structure calculations, making this field an important working tool and bringing together researchers of many different domains.

physics.chem-ph↗

arXiv2015

MADNESS: A Multiresolution, Adaptive Numerical Environment for Scientific Simulation

MADNESS (multiresolution adaptive numerical environment for scientific simulation) is a high-level software environment for solving integral and differential equations in many dimensions that uses adaptive and fast harmonic analysis methods with guaranteed precision based on multiresolution analysis and separated representations. Underpinning the numerical capabilities is a powerful petascale parallel programming environment that aims to increase both programmer productivity and code scalability. This paper describes the features and capabilities of MADNESS and briefly discusses some current applications in chemistry and several areas of physics.

cs.MS↗

arXiv2015

Fragment Approach to Constrained Density Functional Theory Calculations using Daubechies Wavelets

In a recent paper we presented a linear scaling Kohn-Sham density functional theory (DFT) code based on Daubechies wavelets, where a minimal set of localized support functions is optimized in situ and therefore adapted to the chemical properties of the molecular system. Thanks to the systematically controllable accuracy of the underlying basis set, this approach is able to provide an optimal contracted basis for a given system: accuracies for ground state energies and atomic forces are of the same quality as an uncontracted, cubic scaling approach. This basis set offers, by construction, a natural subset where the density matrix of the system can be projected. In this paper we demonstrate the flexibility of this minimal basis formalism in providing a basis set that can be reused as-is, i.e. without reoptimization, for charge-constrained DFT calculations within a fragment approach. Support functions, represented in the underlying wavelet grid, of the template fragments are roto-translated with high numerical precision to the required positions and used as projectors for the charge weight function. We demonstrate the interest of this approach to express highly precise and efficient calculations for preparing diabatic states and for the computational setup of systems in complex environments.

cond-mat.mtrl-sci↗

arXiv2015

Accurate and efficient linear scaling DFT calculations with universal applicability

Density Functional Theory calculations traditionally suffer from an inherent cubic scaling with respect to the size of the system, making big calculations extremely expensive. This cubic scaling can be avoided by the use of so-called linear scaling algorithms, which have been developed during the last few decades. In this way it becomes possible to perform ab-initio calculations for several tens of thousands of atoms or even more within a reasonable time frame. However, even though the use of linear scaling algorithms is physically well justified, their implementation often introduces some small errors. Consequently most implementations offering such a linear complexity either yield only a limited accuracy or, if one wants to go beyond this restriction, require a tedious fine tuning of many parameters. In our linear scaling approach within the BigDFT package, we were able to overcome this restriction. Using an ansatz based on localized support functions expressed in an underlying Daubechies wavelet basis -- which offers ideal properties for accurate linear scaling calculations -- we obtain an amazingly high accuracy and a universal applicability while still keeping the possibility of simulating large systems with only a moderate demand of computing resources.

cond-mat.mtrl-sci↗

arXiv2014

Daubechies Wavelets for Linear Scaling Density Functional Theory

We demonstrate that Daubechies wavelets can be used to construct a minimal set of optimized localized contracted basis functions in which the Kohn-Sham orbitals can be represented with an arbitrarily high, controllable precision. Ground state energies and the forces acting on the ions can be calculated in this basis with the same accuracy as if they were calculated directly in a Daubechies wavelets basis, provided that the amplitude of these contracted basis functions is sufficiently small on the surface of the localization region, which is guaranteed by the optimization procedure described in this work. This approach reduces the computational costs of DFT calculations, and can be combined with sparse matrix algebra to obtain linear scaling with respect to the number of electrons in the system. Calculations on systems of 10,000 atoms or more thus become feasible in a systematic basis set with moderate computational resources. Further computational savings can be achieved by exploiting the similarity of the contracted basis functions for closely related environments, e.g. in geometry optimizations or combined calculations of neutral and charged systems.

cond-mat.mtrl-sci↗

arXiv2013

Ab initio calculations of the optical absorption spectra of C60-conjugated polymer hybrids

A recently developed linear-scaling density-functional theory (LS-DFT) formalism is used to calculate optical absorption spectra of hybrids of C60 and the conjugated polymers poly(para-phenylene) (PPP) and poly(para-phenylene vinylene) (PPV). The use of a LS formalism allows calculations on large systems with realistic proportions of C60, which has been of interest for the use of such materials in photovoltaics. Two different bonding structures are tested for the hybrid PPP and for both systems additional peaks are present in the absorption spectra below the original onset of absorption. By identifying the eigenstates involved in the relevant transitions, a weighted density difference is formed, demonstrating the transfer of charge between the polymer chain and the C60, in agreement with experiment. For the hybrid PPV, no additional peaks are observed in the absorption spectrum.

cond-mat.mtrl-sci↗

arXiv2011

Calculating optical absorption spectra for large systems using linear-scaling density-functional theory

A new method for calculating optical absorption spectra within linear-scaling density-functional theory (LS-DFT) is presented, incorporating a scheme for optimizing a set of localized orbitals to accurately represent unoccupied Kohn-Sham states. Three different schemes are compared and the most promising of these, based on the use of a projection operator, has been implemented in a fully-functional LS-DFT code. The method has been applied to the calculation of optical absorption spectra for the metal-free phthalocyanine molecule and the conjugated polymer poly(para-phenylene). Excellent agreement with results from a traditional DFT code is obtained.

cond-mat.mtrl-sci↗