Performance Analysis of Two Quantum Reaction Dynamics Codes: Time-Dependent and Time-Independent Strategies

The computer simulation of reaction dynamics has nowadays reached a remarkable degree of accuracy. Triatomic elementary reactions are rigorously studied with great detail on a straightforward basis using a considerable variety of Quantum Dynamics computational tools available to the scientific community. In our contribution we compare the performance of two quantum scattering codes in the computation of reaction cross sections of a triatomic benchmark reaction such as the gas phase reaction Ne + H2+ %12. NeH++ H. The computational codes are selected as representative of time-dependent (Real Wave Packet [ ]) and time-independent (ABC [ ]) methodologies. The main conclusion to be drawn from our study is that both strategies are, to a great extent, not competing but rather complementary. While time-dependent calculations advantages with respect to the energy range that can be covered in a single simulation, time-independent approaches offer much more detailed information from each single energy calculation. Further details such as the calculation of reactivity at very low collision energies or the computational effort related to account for the Coriolis couplings are analyzed in this paper

Computational Methods in Science and Engineering : Proceedings of the Workshop SimLabs@KIT, November 29 - 30, 2010, Karlsruhe, Germany

In this proceedings volume we provide a compilation of article contributions equally covering applications from different research fields and ranging from capacity up to capability computing. Besides classical computing aspects such as parallelization, the focus of these proceedings is on multi-scale approaches and methods for tackling algorithm and data complexity. Also practical aspects regarding the usage of the HPC infrastructure and available tools and software at the SCC are presented

The 1999 Center for Simulation of Dynamic Response in Materials Annual Technical Report

Introduction: This annual report describes research accomplishments for FY 99 of the Center for Simulation of Dynamic Response of Materials. The Center is constructing a virtual shock physics facility in which the full three dimensional response of a variety of target materials can be computed for a wide range of compressive, ten- sional, and shear loadings, including those produced by detonation of energetic materials. The goals are to facilitate computation of a variety of experiments in which strong shock and detonation waves are made to impinge on targets consisting of various combinations of materials, compute the subsequent dy- namic response of the target materials, and validate these computations against experimental data

Recent advances in the application of the Schwinger multichannel method with pseudopotentials to electron-molecule collisions

The Schwinger multichannel method [K. Takatsuka and V. McKoy, Phys. Rev. A 30, 1734 (1984)], which is based on the Schwinger variational principle for the scattering amplitude [J. Schwinger, Phys. Rev. 72, 742 (1947)], was designed to account for exchange, polarization and electronically multichannel coupling effects in the low-energy region of electron scattering from molecules with arbitrary geometry. The applications of the method became more ambitious with the availability of computer power combined with parallel processing, use of norm-conserving pseudopotentials and improvement of the description of target excited states (minimal orbital basis for single configuration interaction). The most recent applications involving 33 and 45 electronically open channels for phenol and ethylene molecules, represent good examples of the present status of the method. In this colloquium, we review the strategy and point out new directions to apply the method in its full extension

Methods, software, and benchmarks for modeling long timescale dynamics in solid-state atomic systems

textThe timescale of chemical reactions in solid-state systems greatly exceeds what may be modeled by direct integration of Newton's equation of motion. This limitation spawned the development of many different methods such as (adaptive) kinetic Monte Carlo (A)KMC, (harmonic) transition state theory (H)TST, parallel replica dynamics (PRD), hyperdynamics (HD), and temperature accelerated dynamics. The focus of this thesis was to (1) implement many of these methods in a single open-source software package (2) develop standard benchmarks to compare their accuracy and computational cost and (3) develop new long timescale methods. The lack of a open-source package that implements long timescale methods makes it difficult to directly evaluate the quality of different approaches. It also impedes the development of new techniques. Due to these concerns we developed Eon, a program that implements several long timescale methods including PRD, HD, and AKMC as well as global optimization algorithms basin hopping, and minima hopping. Standard benchmarks to evaluate the performance of local geometry optimization; global optimization; and single-ended and double-ended saddle point searches were created. Using Eon and several other well known programs, the accuracy and performance of different algorithms was compared. Important to this work is a website where anyone may download the code to repeat any of the numerical experiments. A new method for long timescale simulations is also introduced: molecular dynamics saddle search adaptive kinetic Monte Carlo (AKMC-MDSS). AKMC-MDSS improves upon AKMC by using short high-temperature MD trajectories to locate the important low-temperature reaction mechanisms of interest. Most importantly, the use of MD enables the development of a proper stopping criterion for the AKMC simulation that ensures that the relevant reaction mechanisms at the low temperature have been found. Important to the simulation of any material is knowledge of the experimental structure. Extended x-ray absorption fine structure (EXAFS) is a technique often used to determine local atomic structure. We propose a technique to quantitatively measure the accuracy of the commonly used fitting models. This technique reveals that the fitting models interpreted nanoparticles as being significantly more ordered and of much shorter bond length than they really are.Chemistr

GPAW: open Python package for electronic-structure calculations

We review the GPAW open-source Python package for electronic structure calculations. GPAW is based on the projector-augmented wave method and can solve the self-consistent density functional theory (DFT) equations using three different wave-function representations, namely real-space grids, plane waves, and numerical atomic orbitals. The three representations are complementary and mutually independent and can be connected by transformations via the real-space grid. This multi-basis feature renders GPAW highly versatile and unique among similar codes. By virtue of its modular structure, the GPAW code constitutes an ideal platform for implementation of new features and methodologies. Moreover, it is well integrated with the Atomic Simulation Environment (ASE) providing a flexible and dynamic user interface. In addition to ground-state DFT calculations, GPAW supports many-body GW band structures, optical excitations from the Bethe-Salpeter Equation (BSE), variational calculations of excited states in molecules and solids via direct optimization, and real-time propagation of the Kohn-Sham equations within time-dependent DFT. A range of more advanced methods to describe magnetic excitations and non-collinear magnetism in solids are also now available. In addition, GPAW can calculate non-linear optical tensors of solids, charged crystal point defects, and much more. Recently, support of GPU acceleration has been achieved with minor modifications of the GPAW code thanks to the CuPy library. We end the review with an outlook describing some future plans for GPAW