905 research outputs found
Global Minima for Transition Metal Clusters Described by Sutton-Chen Potentials
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.Comment: 16 pages, 7 figures, revte
The Dynamics of Structural Transitions in Sodium Chloride Clusters
In recent experiments on sodium chloride clusters structural transitions
between nanocrystals with different cuboidal shapes were detected. Here we
presents results for the thermodynamics and dynamics of one of these clusters,
(NaCl)35Cl-. As the time scales for the structural transitions can be much
longer than those accessible by conventional dynamics simulations, we use a
master equation to describe the probability flow within a large sample of
potential energy minima. We characterize the processes contributing to
probability flow between the different nanocrystals, and obtain rate constants
and activation energies for comparison with the experimental values.Comment: 11 pages, 8 figures, revte
On the Thermodynamics of Global Optimization
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.Comment: 4 pages, 3 figures, revte
Structural Consequences of the Range of the Interatomic Potential: a Menagerie of Clusters
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.Comment: 26 pages, 16 figures, revtex. Unabridged version of paper accepted by
J. Chem. Soc., Faraday Tran
Rearrangements and Tunneling Splittings in Small Water Clusters
Recent far-infrared vibration-rotation tunneling (FIR-VRT) experiments pose
new challenges to theory because the interpretation and prediction of such
spectra requires a detailed understanding of the potential energy surface (PES)
away from minima. In particular we need a global description of the PES in
terms of a complete reaction graph. Hence all the transition states and
associated mechanisms which might give rise to observable tunneling splittings
must be characterized. It may be possible to guess the detailed permutations of
atoms from the transition state alone, but experience suggests this is unwise.
In this contribution a brief overview of the issues involved in treating the
large amplitude motions of such systems will be given, with references to more
detailed discussions and some specific examples. In particular we will consider
the effective molecular symmetry group, the classification of rearrangement
mechanisms, the location of minima and transition states and the calculation of
reaction pathways. The application of these theories to small water clusters
ranging from water dimer to water hexamer will then be considered. More details
can be found in recent reviews.Comment: 15 pages, 5 figures. This paper was prepared in August 1997 for the
proceedings volume of the NATO-ASI meeting on "Recent Theoretical and
Experimental Advances in Hydrogen Bonded Clusters" edited by Sotiris
Xantheas, which has so far not appeare
Evolution of the Potential Energy Surface with Size for Lennard-Jones Clusters
Disconnectivity graphs are used to characterize the potential energy surfaces
of Lennard-Jones clusters containing 13, 19, 31, 38, 55 and 75 atoms. This set
includes members which exhibit either one or two `funnels' whose low-energy
regions may be dominated by a single deep minimum or contain a number of
competing structures. The graphs evolve in size due to these specific size
effects and an exponential increase in the number of local minima with the
number of atoms. To combat the vast number of minima we investigate the use of
monotonic sequence basins as the fundamental topographical unit. Finally, we
examine disconnectivity graphs for a transformed energy landscape to explain
why the transformation provides a useful approach to the global optimization
problem.Comment: 13 pages, 8 figures, revte
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