arXiv subjects
Publications and source records attributed to Jonathan Doye.
Using a Monte Carlo minimization approach we report the global minima for metal clusters modelled by the Sutton-Chen family of potentials with N<81, where N is the number of atoms. The resulting structures are discussed in the light of both experimental and theoretical data for clusters of the appropriate elements.
We have attempted to find the global minima of clusters containing between 20 and 80 atoms bound by the Morse potential as a function of the range of the interatomic force. The effect of decreasing the range is to destabilize strained structures, and hence the global minimum changes from icosahedral to decahedral to face-centred-cubic as the range is decreased. For N>45 the global minima associated with a long-ranged potential have polytetrahedral structures involving defects called disclination lines. For the larger clusters the network of disclination lines is disordered and the global minimum has an amorphous structure resembling a liquid. The size evolution of polytetrahedral packings enables us to study the development of bulk liquid structure in finite systems. As many experiments on the structure of clusters only provide indirect structural information, these results will be very useful in aiding the interpretation of experiment. They also provide candidate structures for theoretical studies using more specific and computationally expensive descriptions of the interatomic interactions. Furthermore, Morse clusters provide a rigorous testing ground for global optimization methods.
We have studied the thermodynamics of isolated homopolymer chains of varying stiffness using a lattice model. A complex phase behaviour is found; phases include chain-folded `crystalline' structures, the disordered globule and the coil. It is found, in agreement with recent theoretical calculations, that the temperature at which the solid-globule transition occurs increases with chain stiffness, whilst the $θ$-point has only a weak dependence on stiffness. Therefore, for sufficiently stiff chains there is no globular phase and the polymer passes directly from the solid to the coil. This effect is analogous to the disappearance of the liquid phase observed for simple atomic systems as the range of the potential is decreased.
Theoretical design of global optimization algorithms can profitably utilize recent statistical mechanical treatments of potential energy surfaces (PES's). Here we analyze a particular method to explain its success in locating global minima on surfaces with a multiple-funnel structure, where trapping in local minima with different morphologies is expected. We find that a key factor in overcoming trapping is the transformation applied to the PES which broadens the thermodynamic transitions. The global minimum then has a significant probability of occupation at temperatures where the free energy barriers between funnels are surmountable.