Simonetta Iarlori

Self-trapping vs. non-trapping of electrons and holes in organic insulators: polyethylene

We show, by electronic structure based molecular dynamics simulations, that an extra electron injected in crystalline polyethylene should fall spontaneously into a self-trapped state, a shallow donor with a large novel distortion pattern involving a pair of trans-gauche defects. Parallel calculations show instead that a hole will remain free and delocalized. We trace the difference of behavior to the intrachain nature of the hole, as opposed to the interchain one of the electron, and argue that applicability of this concept could be more general. Thus electrons (but not holes) should tend to self-trap in saturated organic insulators, but not for example in aromatic insulators, where both carriers are intrachain.

Role of layering oscillations at liquid metal surfaces in bulk recrystallization and surface melting.

The contrasting melting behavior of different surface orientations in metals can be explained in terms of a repulsive or attractive effective interaction between the solid-liquid and the liquid-vapor interface. We show how a crucial part of this interaction originates from the layering effects near the liquid metal surface. Its sign depends on the relative tuning of layering oscillations to the crystal interplanar spacing, thus explaining the orientational dependence. Molecular dynamics recrystallization simulations of Au surfaces provide direct and quantitative evidence of this phenomenon.

AB INITIO MOLECULAR DYNAMICS STUDY OF METALLOCENE CATALYSED ETHYLENE POLYMERIZATION. III. BIS- VERSUS MONO-CYCLOPENTADIENYL METALLOCENES

Ab initio molecular dynamics simulations of a catalytic reaction are presented. Ethylene insertion is observed to proceed in a SiH2-bridged di-(cyclopentadienyl)methylzirconocene cation as well as in the corresponding titanocene cation. The entire reaction path starting from the π coordinated ethylene-metallocene complex up to and including propyl formation takes place in about 150 fs for the zirconocene and 500 fs for the titanocene. For both a Zr and a Ti based monocyclopentadienyl metallocene no insertion was observed, which is indicative of a barrier to insertion. In accordance with this, stepwise variation of the relevant C-C distance along the reaction path as set prior to the dynamics simulation did reveal insertion. These data are in qualitative agreement with results obtained from static simulations on similar species.

Ab initio molecular dynamics study of metallocene-catalysed ethylene polymerisation: temperature effects

Abstract The feasibility of using ab initio molecular dynamics simulations to study chemical reactions is illustrated by further studies on ethylene insertion into a bridged bis (cyclopentadienyl) methylzirconocene. Previous observation that the insertion is extremely fast, suggesting the absence of an energetic barrier, is further supported by results obtained from simulations at various temperatures. In view of the short time span (170 fs) for insertion, coupling to an external heat bath cannot be justified. Therefore, a microcanonical molecular dynamics simulation starting at T = 0 K was performed. This simulation also revealed extremely fast ethylene insertion, which was accompanied by a temperature rise of a few hundred kelvin.

Molecular Dynamics Simulations of Metal Surfaces: Surface Melting and Non-Melting, and Tip-Surface Interactions

Molecular dynamics simulations have been used in order to gain some insight into two problems of interest to this workshop.

Reconstruction of the diamond (1 1 1) surface

Abstract We have fully characterized the (2 × 1) reconstruction of the diamond (1 1 1) surface via first priciples molecular dynamics simulations. We find that the reconstructed geometry consists of dimerized π-bonded chains. The dimerization parameter is 1.4% and there is no buckling. We also find multilayer relaxations and a great stability of the reconstructed geometry against thermal fluctuations up to ⋍2000 K.