Andrew Canning

Extended Si |P[311|P] defects

We perform total energy calculations based on the tight-binding Hamiltonian scheme (i) to study the structural properties and energetics of the extended {311} defects depending upon their dimensions and interstitial concentrations and (ii) to find possible mechanisms of interstitial capture by and release from the {311} defects. The generalized orbital-based linear-scaling method implemented on Cray-T3D is used for supercell calculations of large scale systems containing more than 1000 Si atoms.

Carbon Superatom Thin Films

We report on quantum molecular dynamics simulations of C{sub 28} deposition on a semiconducting surface. Our results show that under certain deposition conditions C{sub 28} {close_quote}s act as building blocks on a nanometer scale to form a thin film of nearly defect-free molecules. The C{sub 28} {close_quote}s behave as carbon superatoms, with the majority of them being threefold or fourfold coordinated, similar to carbon atoms in amorphous systems. The microscopic structure of the deposited film supports recent suggestions about the stability of a new form of carbon, the hyperdiamond solid. {copyright} {ital 1997} {ital The American Physical Society}

O(N) tight-binding molecular dynamics on massively parallel computers: an orbital decomposition approach

The implementation of an O(N) tight-binding molecular dynamics code on the Cray T3D parallel computer is discussed. The O(N) energy functional depends on non-orthogonal, localised orbitals and a chemical potential parameter which determines the number of electrons in the system. The localisation introduces a sparse nature to the orbital data and Hamiltonian matrix, greatly changing the coding on parallel machines compared to non-localised systems. The data distribution, communication routines and dynamic load-balancing scheme of the program are presented in detail together with the speed and scaling of the code on various homogeneous and inhomogeneous physical systems. Performance results will be presented for systems of 2048 to 32768 atoms on 32 to 512 processors. We discuss the relevance to quantum molecular dynamics simulations with localised orbitals, of techniques used for programming short-range classical molecular dynamics simulations on parallel machines. The absence of global communications and the localised nature of the orbitals makes these algorithms extremely scalable in terms of memory and speed on parallel systems with fast communications. The main aim of this article is to present in detail all the new concepts and programming techniques that localisation of the orbitals introduces which scientists, coming from a background in non-localised quantum molecular dynamics simulations, may be unfamiliar with.

DISORDERED AND ORDERED C28 SOLIDS

Using tight-binding molecular dynamics, we have performed computer experiments to mimic the gas phase growth of a disordered solid composed of C28 fullerenes. The growth has been simulated by repeated low energy collisions of molecules coming from random directions. The resulting solid is composed of undamaged C28 cages, with most fullerenes being three- and four-fold coordinated, similar to C atoms in amorphous materials. The system contains a high percentage of distorted sp2 C sites and only a small proportion of sp3 sites. These results help clarify the structure of disordered films obtained experimentally by small fullerene deposition on surfaces. Furthermore, we have compared the properties of the disordered C28 solid (a-C28) with those of ordered C28 solids. We have found that the energy of a-C28 is close to that of hyperdiamond (0.1 eV/atom higher) and differs by a few meV from that of other ordered structures, such as 2D-hypergraphite, hexagonal and clathrate solids. This indicates that in condens...

Graphitization of diamond (111) studied by first principles molecular dynamics

Abstract Large scale first principles numerical simulations, performed on modern massively parallel computers, can be usefully applied to study the physics of semiconductor surface and interface systems. We report on a recent study of the surface-initiated diamond to graphite structural transition of crystalline carbon. Our investigation consisted of a series of fully ab initio molecular dynamic simulations of the diamond C(111)-(2 sx 1) surface, with cells containing from 200 to 300 atoms. We observed a spontaneous graphitization of the surface, followed by a fast graphitization of the entire diamond slab, at temperatures above 2500 K. We find that the transition starts at the reconstructed surface layer and rapidly proceeds into the bulk region by highly correlated breaking of z -oriented diamond bonds. We identify a precursor seed to the structural transformation, and in particular we obtain a non abrupt graphite-diamond interface forming prior to the transition. This interface is characterised by a regular alternation of three- and four-fold coordinated atoms along the [110] direction at the convex corner of the phase boundary. Local density of states (LDOS) analysis reveals the presence of chemically active sites at the interface region. Our results are in agreement with experiments on the thermal behaviour of diamond (111), confirm early measurements about surface induced graphitization of diamond, and bear important implications to the formation process of graphite islands in chemical vapor deposited (CVD) diamond films. In particular, we discuss the role of surface dangling bonds as chemisorption sites for atomic hydrogen, in relation to the stabilisation of CVD-grown diamond films by selective etching.

Quantum Simulations Using Linear Scaling Methods: Clusters on Surfaces

The basic features of orbital based linear scaling methods for electronic structure calculations and molecular dynamics simulations are discussed, and some applications of these approaches within a tight-binding formulation are briefly reviewed. In particular, two studies of clusters on surfaces are presented.