Theory for New Carbon-Based Materials
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THEORY FOR NEW CARBON-BASED MATERIALS D.W. BRENNER, R.C. MOWREY, 3W. MINTMIRE, J.A. HARRISON, D.H. ROBERTSON, M. LYONS, B.I. DUNLAP, AND C.T. WHITE Code 6179, Naval Research Laboratory, Washington, DC. 20375-5000
ABSTRACT We review results of our local-density-functional-based cluster calculations and molecular dynamics simulations of fullerenes and related structures. These include predictions of cohesive energies, electronic structures, and photoelectron spectra for a number of pure and chemically substituted fullerenes, studies of the resilience of C6 0 under severe compression and during surface collisions, simulations of the trapping of He in the interior of C60 , predictions of the strain energy, electronic and elastic properties of graphitic tubules, and simulations of the folding and curling of graphitic ribbons.
INTRODUCTION Techniques for producing macroscopic quantities of C6 o and related fullerene structures have moved carbon-based systems to the forefront of materials research. Crucial to taking full advantage of these potential new materials is an understanding of their atomic-scale structure and properties. To this end we have been examining a broad range of structural, mechanical, chemical and electronic properties of fullerenes and related structures using density-functional-based cluster calculations and molecular dynamics simulations. In this paper we review the results related to fullerenes and related structures generated by the Theoretical Chemistry Section at the Naval Research Laboratory. We begin by giving a brief outline of the theoretical methods that have been used. These include firstprinciples local-density functional (LDF) methods for calculating energetics, electronic structure and photoelectron cross sections, as well as our empirical many-body potential used in the molecular-dynamics simulations. We then review our results related to a number of pure and chemically-substituted fullerenes. This is followed by some very recent results regarding both the electronic and structural properties of nanometer-scale graphitic tubules. We then review our calculations of photoelectron cross sections. Finally, we discuss the molecular dynamics simulations which include simulations of C6 0 under severe compression, the trapping of He inside of C6 0 and the folding and curling of graphitic ribbons.
THEORETICAL METHODS Cluster-Based Local-Density Functional Methods We calculate the LDF electronic structure and total energy using molecular orbitals constructed from linear combinations of Gaussian-type functions. Within this approach the electron repulsion and exchange-correlation potentials are evaluated using a variational fitting of the charge density [1] and orbital and potential fitting basis sets constructed from products of solid-spherical harmonics and Gaussian functions [2]. Maximal use of symmetry has been incorporated into a computer code [3] capable of calculating both the total energy as well as the one-electron energies and wavefunctions. The LDF used is the Perdew-Zunger [4] fit o
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