Cumulant expansion analysis of thermal disorder in face centred cubic copper metal by molecular dynamics simulation

• Published in: Molecular physics Vol. 91; no. 2; pp. 357 - 370
• Main Authors: EDWARDS, A. BRYAN, TILDESLEY, D. J.
• Format: Journal Article
• Language: English
• Published: Taylor & Francis Group 01.06.1997
• ISSN: 0026-8976; 1362-3028
• DOI: 10.1080/002689797171643

Molecular dynamics, using a previously validated semi-empirical potential, are used to explore thermal disorder in face centred cubic copper over the temperature range 100 K to 1100 K. The mean-square displacement (MSD) of atoms from their mean positions has been extracted from the simulation. Results compare favourably with experiment and are consistent with the predictions of a range of theoretical lattice dynamics models. The temperature dependence of the first four cumulants of each probability distribution function (PDF) for the four nearest neighbour shells is compared with the predictions of a simple one-dimensional anharmonic oscillator model. With the exception of the third cumulant for the second shell, the behaviour of the simulated cumulants is consistent with this model. Where direct comparison of the simulation and experimentally derived cumulants is possible the two are found to be in good agreement except at low temperatures where quantum effects (neglected in the classical molecular dynamics simulation) are most significant. The simulated PDF profiles are reproduced well across the temperature range by a reconstruction formula employing only the first four cumulants. The linear expansivitity calculated from the cumulants is in only modest agreement with experimental data. Analysis of the PDF of a shell in the high essentially uncorrelated isotropic vibration of the atoms, but demonstrates the difficulty of extracting MSD values from extended X-ray absorption fine structure (EXAFS) spectroscopy and correlating EXAFS and X-ray diffraction thermal parameters. r limit is consistent with