Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Fast parallel algorithms for short-range molecular dynamics

Owing to the increasing emissions of carbon dioxide (CO2), human life and the ecological environment have been affected by global warming and climate changes. To mitigate the concentration of CO2 in the atmosphere various strategies have been implemented such as separation, storage, and utilization of CO2. Although it has been explored for many years, hydrogenation reaction, an important representative among chemical conversions of CO2, offers challenging opportunities for sustainable development in energy and the environment. Indeed, the hydrogenation of CO2 not only reduces the increasing CO2 buildup but also produces fuels and chemicals. In this critical review we discuss recent developments in this area, with emphases on catalytic reactivity, reactor innovation, and reaction mechanism. We also provide an overview regarding the challenges and opportunities for future research in the field (319 references).

Recent advances in catalytic hydrogenation of carbon dioxide

인공고관절의 세라믹 볼 헤드의 기계적 안정성 평가를 위한 3 차원 유한요소 해석

Starting with coal, followed by petroleum oil and natural gas, the utilization of fossil fuels has allowed the fast and unprecedented development of human society. However, the burning of these resources in ever increasing pace is accompanied by large amounts of anthropogenic CO2 emissions, which are outpacing the natural carbon cycle, causing adverse global environmental changes, the full extent of which is still unclear. Even through fossil fuels are still abundant, they are nevertheless limited and will, in time, be depleted. Chemical recycling of CO2 to renewable fuels and materials, primarily methanol, offers a powerful alternative to tackle both issues, that is, global climate change and fossil fuel depletion. The energy needed for the reduction of CO2 can come from any renewable energy source such as solar and wind. Methanol, the simplest C1 liquid product that can be easily obtained from any carbon source, including biomass and CO2, has been proposed as a key component of such an anthropogenic carbon cycle in the framework of a “Methanol Economy”. Methanol itself is an excellent fuel for internal combustion engines, fuel cells, stoves, etc. It's dehydration product, dimethyl ether, is a diesel fuel and liquefied petroleum gas (LPG) substitute. Furthermore, methanol can be transformed to ethylene, propylene and most of the petrochemical products currently obtained from fossil fuels. The conversion of CO2 to methanol is discussed in detail in this review.

Recycling of carbon dioxide to methanol and derived products – closing the loop

We present a comprehensive mean-field microkinetic model for the methanol synthesis and water-gas-shift (WGS) reactions that includes novel reaction intermediates, such as formic acid (HCOOH) and hydroxymethoxy (CH3O2) and allows for the formation of formic acid (HCOOH), formaldehyde (CH2O), and methyl formate (HCOOCH3) as byproducts. All input model parameters were initially derived from periodic, self-consistent, GGA-PW91 density functional theory calculations on the Cu(111) surface and subsequently fitted to published experimental methanol synthesis rate data, which were collected under realistic conditions on a commercial Cu/ZnO/Al2O3 catalyst. We find that the WGS reaction follows the carboxyl (COOH)-mediated path and that both CO and CO2 hydrogenation pathways are active for methanol synthesis. Under typical industrial methanol synthesis conditions, CO2 hydrogenation is responsible for ∼2/3 of the methanol produced. The intermediates of the CO2 pathway for methanol synthesis include HCOO*, HCOOH*, C...

Mechanism of Methanol Synthesis on Cu through CO2 and CO Hydrogenation

Combined computational and experimental investigation of the refractory properties of La2Zr2O7

Structure and thermodynamics of pure cubic ZrO2 and HfO2 were studied computationally and experimentally from their tetragonal to cubic transition temperatures (2311 and 2530 °C) to their melting points (2710 and 2800 °C). Computations were performed using automated ab initio molecular dynamics techniques. High temperature synchrotron X-ray diffraction on laser heated aerodynamically levitated samples provided experimental data on volume change during tetragonal-to-cubic phase transformation (0.55 ± 0.09% for ZrO2 and 0.87 ± 0.08% for HfO2), density and thermal expansion. Fusion enthalpies were measured using drop and catch calorimetry on laser heated levitated samples as 55 ± 7 kJ/mol for ZrO2 and 61 ± 10 kJ/mol for HfO2, compared with 54 ± 2 and 52 ± 2 kJ/mol from computation. Volumetric thermal expansion for cubic ZrO2 and HfO2 are similar and reach (4 ± 1)·10-5/K from experiment and (5 ± 1)·10-5/K from computation. An agreement with experiment renders confidence in values obtained exclusively from computation: namely heat capacity of cubic HfO2 and ZrO2, volume change on melting, and thermal expansion of the liquid to 3127 °C. Computed oxygen diffusion coefficients indicate that above 2400 °C pure ZrO2 is an excellent oxygen conductor, perhaps even better than YSZ.

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Combined computational and experimental investigation of high temperature thermodynamics and structure of cubic ZrO2 and HfO2

We present ab initio calculations of the phase diagram and the equation of state of Ta in a wide range of volumes and temperatures, with volumes from 9 to 180 A^3/atom, temperature as high as 20 000 K, and pressure up to 7 Mbars. The calculations are based on first principles, in combination with techniques of molecular dynamics, thermodynamic integration, and statistical modeling. Multiple phases are studied, including the solid, fluid, and gas single phases, as well as two-phase coexistences. We calculate the critical point by direct molecular dynamics sampling, and extend the equation of state to very low density through virial series fitting. The accuracy of the equation of state is assessed by comparing both the predicted melting curve and the critical point with previous experimental and theoretical investigations.

Equation of state of solid, liquid and gaseous tantalum from first principles

Although various approaches for melting point calculations from first principles have been proposed and employed for years, their practical implementation has hitherto remained a complex and time-consuming process. The SLUSCHI code (Solid and Liquid in Ultra Small Coexistence with Hovering Interfaces) drastically simplifies this procedure into an automated package, by implementing the recently-developed small-size coexistence method and putting together a series of steps that lead to final melting point evaluation. Based on density functional theory, SLUSCHI employs Born–Oppenheimer molecular dynamics techniques under the isobaric–isothermal ( NPT ) ensemble, with interface to the first-principles code VASP . In order to make this useful code available to a wide community of researchers who could benefit from it, this paper outlines the procedure to perform melting point calculations, and presents a detailed user guide to the code.

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A user guide for SLUSCHI: Solid and Liquid in Ultra Small Coexistence with Hovering Interfaces

We propose a scheme that drastically improves the efficiency of Widom's particle insertion method by efficiently sampling cavities while calculating the integrals providing the chemical potentials of a physical system. This idea enables us to calculate chemical potentials of liquids directly from first-principles without the help of any reference system, which is necessary in the commonly used thermodynamic integration method. As an example, we apply our scheme, combined with the density functional formalism, to the calculation of the chemical potential of liquid copper. The calculated chemical potential is further used to locate the melting temperature. The calculated results closely agree with experiments.

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Qi-Jun Hong

Papers

Combined computational and experimental investigation of the refractory properties of La2Zr2O7

Combined computational and experimental investigation of high temperature thermodynamics and structure of cubic ZrO2 and HfO2

Equation of state of solid, liquid and gaseous tantalum from first principles

A user guide for SLUSCHI: Solid and Liquid in Ultra Small Coexistence with Hovering Interfaces

Direct first-principles chemical potential calculations of liquids.