30 research outputs found
New Tetrahedral Global Minimum for the 98-atom Lennard-Jones Cluster
A new atomic cluster structure corresponding to the global minimum of the
98-atom Lennard-Jones cluster has been found using a variant of the
basin-hopping global optimization algorithm. The new structure has an unusual
tetrahedral symmetry with an energy of -543.665361, which is 0.022404 lower
than the previous putative global minimum. The new LJ_98 structure is of
particular interest because its tetrahedral symmetry establishes it as one of
only three types of exceptions to the general pattern of icosahedral structural
motifs for optimal LJ microclusters. Similar to the other exceptions the global
minimum is difficult to find because it is at the bottom of a narrow funnel
which only becomes thermodynamically most stable at low temperature.Comment: 3 pages, 2 figures, revte
Global Optimization by Basin-Hopping and the Lowest Energy Structures of Lennard-Jones Clusters Containing up to 110 Atoms
We describe a global optimization technique using `basin-hopping' in which
the potential energy surface is transformed into a collection of
interpenetrating staircases. This method has been designed to exploit the
features which recent work suggests must be present in an energy landscape for
efficient relaxation to the global minimum. The transformation associates any
point in configuration space with the local minimum obtained by a geometry
optimization started from that point, effectively removing transition state
regions from the problem. However, unlike other methods based upon hypersurface
deformation, this transformation does not change the global minimum. The lowest
known structures are located for all Lennard-Jones clusters up to 110 atoms,
including a number that have never been found before in unbiased searches.Comment: 8 pages, 3 figures, revte
Cooking the Books: The Ethical Implication of Performance Management on Timely Services for Veterans
The integrity of the performance reform movement is vulnerable to manipulation because of strains on the principal–agent relationship, yet the quality of performance reforms has been understudied. This case study assesses performance reforms by examining the Veterans Health Administration’s development and implementation of a measure of timely medical services for veterans. The analysis reveals that ambitious goals and underdeveloped training programs contributed to unethical actions and data manipulation. Investments in elements crucial to a successful principal–agent relationship and performance measures accounting for ethical behavior could result in more meaningful uses of performance reforms
The maximum fluidity length of solidifying Sn-Cu-Ag-Ni solder alloys
During wave soldering, it is important that a solder is able to flow easily to fill joints and to drain away to leave tidy fillets. The maximum fluidity length (L ) is a simple measure of the flow behavior of solidifying alloys, defined as the distance a cooling and solidifying alloy can flow in a constant cross-section before the developing microstructure arrests flow. This paper explores the influence of alloy composition on L in Sn-rich Sn-Cu-Ag-Ni alloys with compositions relevant to wave soldering. Significant differences in L are measured among candidate lead-free solder alloys, which are discussed with respect to the phase diagrams and the mode of solidification