Embedded Cluster Model: Application to Molecular Crystals
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Embedded Cluster Model: Application to Molecular Crystals Maija M. Kuklja1, Frank J. Zerilli2, and Peter Sushko3 1
Division of Materials Research, National Science Foundation, Arlington, VA 22230 Naval Surface Warfare Center Indian Head Division, Indian Head, MD 20640, USA 3 Department of Physics and Astronomy, University College London, London, WC1E 6BT, UK 2
ABSTRACT Multiscale modeling using an embedded cluster approach is presented and applied to study the structure and properties of molecular crystals. We discuss the results of hydrostatic compression modeling of 1,1-diamino-2,2-dinitroethylene obtained with the embedded cluster model and the Hartree-Fock method and compare these with the full periodic crystal structure calculations. Details of the electronic structure of the perfect, highly compressed material are discussed. The results demonstrate the applicability of the embedded cluster model. We show that the band gap of the perfect material is not sensitive to hydrostatic compression, but some changes induced by the pressure take place in the valence band.
INTRODUCTION In recent years, there has been a great deal of research in ab initio calculations for complex solid materials with emphasis on the nanoscale aspects of their properties and behavior. In spite of the fact that during the last decade significant progress has been achieved in simulations of the equation of state (EOS) and the electronic structure of energetic crystals, many questions still remain unanswered. Critical issues for modern detonation theory are to understand, create models for, and simulate the behavior of both small and large-scale structures and to make the connection between material structure, properties, and functions. There is a need for a multidisciplinary and system-oriented approach for the development of more generic models and simulation methods, achieved through the cross-fertilization of ideas across disciplines and the systematic flow of information among different research fields. Previously we have studied the electronic structure of perfect and defective RDX (C3H6N6O6), PETN (C5H8N4O12), FOX-7 (C2H4N4O4), and some other energetic materials under shock conditions. The theoretical approach used is based on ab initio Hartree-Fock calculations employing both periodic structure and molecular cluster models. The results of computer simulations of various structural defects1,2,3,4,5,6,7,8 in RDX crystals under ambient and shock conditions were summarized in recent reviews9,10. In particular, we found that formation energy for vacancies2 and vacancy complexes3 is better reproduced with the molecular cluster model than with the periodic structure model due to strong unphysical interactions between periodically repeated defects in the crystal lattice. The details of the electronic structure, however, are better described by the periodic model due to the absence of a crystal field around the molecular cluster and the neglect of lattice polarization in the molecular cluster model. Interesting conclusions follow fr
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