摘要:

The combination of developments in theoretical numerical and computational techniques has made it possible to study a wide range of scientific problems from first principles. In this paper I shall review the capabilities of first principles calculations, highlighting both their strengths and weaknesses. ©1999 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.59471

出版商:AIP    

年代:1999

数据来源: AIP

摘要:

Focusing on molecular electronic structure, an outline is presented of the range of methods commonly used in computational chemistry, with consideration given to the accuracy, limitations and performance costs of the HF, DFT, MP2 and CCSD methods in understanding neutron-scattering related phenomena. Using GAMESS-UK as an example of a large electronic structure package, we illustrate the capabilities of these methods in computing the geometrical structures of transition metal complexes. These studies both reinforce the value of DFT, and reveal an overall inconsistency in the MP2-based predictions. Finally, emphasis is given to the cost effectiveness and applicability of the spectrum of hardware available to the computational chemist, from the single-user PC to massively parallel processors (MPP). We illustrate the potential of the latter platforms in enhancing the level of desk-top simulations by two-orders of magnitude, and demonstrate the potential of parallel PC-based Beowulf Systems. ©1999 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.59480

出版商:AIP    

年代:1999

数据来源: AIP

摘要:

We describe the development of reverse Monte Carlo methods for modelling structural disorder in crystalline materials from neutron powder diffraction data, explaining the advantages and disadvantages of the different methods available. The distinction between elastic (Bragg) scattering and total scattering measurements, and the different type of structural information that can be deduced from them, is made in some detail. The possibilities for further developments are described. ©1999 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.59467

出版商:AIP    

年代:1999

数据来源: AIP

摘要:

Free energy minimization, using analytical derivatives, has been applied to three different cubic zeolite structures including faujasite which experimentally has been demonstrated to show uniform negative thermal expansion over a wide temperature range. Both total and cell-only free energy minimisation have been examined for three different interatomic potential sets. For all models the former method is found to breakdown close to room temperature and therefore the cell-only approach is the method of preference. ©1999 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.59485

出版商:AIP    

年代:1999

数据来源: AIP

摘要:

We present a correction procedure to account for multiple scattering effects in Deep Inelastic Neutron Scattering experiments in molecular systems, based on a Monte Carlo simulation in which the energy-transfers are described with a synthetic model. Experimental results in polyethylene samples of different thicknesses performed at the Bariloche LINAC are presented along with numerical simulations that show a good agreement between experiment and calculations. It is shown that multiple scattering effects may cause an important non-symmetric effect on the observed peaks. ©1999 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.59470

出版商:AIP    

年代:1999

数据来源: AIP

摘要:

A brief description of the Neutron Instrument Simulation Package (NISP) is given, together with an example of its use to calculate the resolution function of an inelastic spectrometer. ©1999 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.59486

出版商:AIP    

年代:1999

数据来源: AIP

摘要:

We present a new approach to multiple scattering correction, based on a combination of taking experimental data at several incident neutron wavelengths and an iterative numerical evaluation of multiple scattering contributions. It allowed us to model independently determine the low intensity part of dynamic structure factor in a typical glassCa0.4K0.6(NO3)1.4.The new method makes the range of small momentum transfer in liquid and amorphous systems accessible for neutron inelastic scattering investigation and opens up new opportunities for the study of dynamic processes in these systems. ©1999 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.59487

出版商:AIP    

年代:1999

数据来源: AIP

摘要:

Structural properties of glasses in general, and complicated multi component glasses in particular, are difficult to extract directly from experimental results. This is partly because global structural investigations by diffraction techniques are only providing a one-dimensional average representation of the structure, usually without any information of partial pair correlations for a single experiment. Furthermore, many glasses are so chemically complicated that it has, so far, been difficult to develop interatomic model potentials between the particles that are accurate enough for reproducing the diffraction data using conventional simulation techniques such as molecular dynamics (MD) and Monte Carlo (MC) simulations. The reverse Monte Carlo (RMC) modelling method overcomes the problem of finding model potentials and instead it makes direct use of the available experimental data. In this way, the RMC method produces three dimensional structural models which are quantitatively consistent with the available data and the applied constraint, e.g. constraints of density, minimum atomic sizes and coordination numbers. As an illustrative example, we show how the RMC method has been used for structural modelling of the fast ion conducting glasses(AgI)0.6(Ag2O-2B2O3)0.4and(AgI)0.75(Ag2MoO4)0.25.Some advantages and limitations of the method will also be discussed. ©1999 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.59472

出版商:AIP    

年代:1999

数据来源: AIP

摘要:

In our computer simulation the preparation process was similar to the atom-by-atom deposition onto a substrate which is a standard procedure to prepare amorphous carbon. For the calculation of the interactions between carbon atoms a tight-binding potential was used. All parameters and functions of the potential were fitted to the results of local density functional calculations for graphite, diamond and linear chains. The Newton equations of motion were solved by the Verlet algorithm. The amorphous structures with fivefold, sixfold and sevenfold rings and chain-like atomic arrangement were obtained on diamond (111) surface. ©1999 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.59473

出版商:AIP    

年代:1999

数据来源: AIP

摘要:

We are interested in the thermodynamics of the gas filling of clathrate hydrates. In order to determine the pressure-dependent filling of the cages, neutron powder diffraction experiments onN2andCO2clathrates were performed. Interaction potentials were refined by comparing the experimentally determined fillings with those generated by MC-sorption calculations. Unsatisfactory agreement between experiment and simulation is observed when using the widely employed SPC water-water interaction potential. ©1999 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.59474

出版商:AIP    

年代:1999

数据来源: AIP