ABSTRACT
Understanding the structure of bimetallic clusters is increasingly important due to their emerging practical applications. Herein we investigate the structure of 38-atom CuPd clusters using a genetic algorithm with cluster energies described by the semi-empirical Gupta potential. Selected clusters are then refined with density functional theory. Three different parameterisations of the Gupta potential are used and their performance assessed to understand what features of bulk and surfaces are necessary to capture for accurate description of small clusters. Three general regions of motif stability exist; for the Pd majority clusters (Pd38 to Cu4Pd34) the truncated octahedron is most stable, while for clusters of intermediate compositions (Cu5Pd33 to Cu25Pd13) a "pancake" icosahedron is most stable, and for the Cu majority clusters (Cu26Pd12 to Cu38) again the truncated octahedron is most stable. CuPd clusters tend to segregate to a Cu-core, Pd-shell structure if possible, and at higher Cu compositions, the Pd segregates to the faces of the cluster. Using multiple parameterisations of the Gupta potential ensures the full variety of possible structures is found, and improves the search for the most stable CuPd clusters.
ABSTRACT
A structural comparison method (SCM) was created to quantify the structural diversity of nanoclusters and was implemented into a global optimization algorithm to evaluate structural diversity between generated clusters on the fly and promote exploration of the potential energy surface. The SCM evaluated topological differences between clusters using the common neighbor analysis and provided a numerical measure of similarity between the two clusters. The SCM was implemented into a genetic algorithm by integrating it into a new structure + energy fitness operator such that structural diversity of clusters in the population and their energies were used to assign fitness values to clusters. The efficiency of the genetic algorithm with this new fitness operator was benchmarked against several Lennard-Jones clusters (LJ38, LJ75, and LJ98) known to be difficult cases for global optimization algorithms. For LJ38 and LJ75, this new structure + energy fitness operator performed equally well or better than the energy fitness operator. However, the efficiency of locating the global minimum of LJ98 decreased using this new structure + energy fitness operator. Further analysis of the genetic algorithm with this fitness operator showed that the algorithm did indeed promote exploration of the PES of LJ98 as desired but hindered refinement of clusters, preventing it from locating the global minimum even if the energy funnel of the global minimum had been located.
Subject(s)
Algorithms , Physical Phenomena , ThermodynamicsABSTRACT
BACKGROUND: Up to a quarter of people in the UK with a diagnosis of dementia are prescribed an antipsychotic in any year. The potential risks of such treatment are becoming clearer, but the benefits remain uncertain. Concern about the frequency and quality of such prescribing was expressed in the National Dementia Strategy for England in 2009. AIMS: To provide an estimate of the prevalence of antipsychotic use for dementia in secondary mental health services in the UK and to collect data relevant to quality improvement initiatives for such prescribing practice. METHOD: In the context of a UK quality improvement programme, relevant clinical audit data were collected for patients with dementia under the care of specialist older people's mental health services. RESULTS: Fifty-four mental health National Health Service (NHS) trusts submitted data on 10 199 patients. Of those patients without comorbid psychotic illness, 1620 (16%) were prescribed an antipsychotic; the common clinical indications for such medication were agitation, psychotic symptoms, aggression and distress. Multivariable regression found younger age, care home or in-patient setting, vascular or Parkinson's disease dementia and greater severity of dementia to be all significantly associated with being prescribed antipsychotic medication. Of the 1001 (62%) patients prescribed treatment for more than 6 months, only three-quarters had a documented review of therapeutic response in the previous 6 months. CONCLUSIONS: The data reveal areas of relatively good current practice, including consideration of alternatives to antipsychotic medication and clear documentation of target symptoms. They also suggest areas for improvement, such as the frequency and quality of review of long-term medication. Strategies to reduce antipsychotic use should take account of the demographic and clinical variables predicting increased likelihood of antipsychotic prescription.
