28,726 research outputs found
First-principles molecular structure search with a genetic algorithm
The identification of low-energy conformers for a given molecule is a
fundamental problem in computational chemistry and cheminformatics. We assess
here a conformer search that employs a genetic algorithm for sampling the
low-energy segment of the conformation space of molecules. The algorithm is
designed to work with first-principles methods, facilitated by the
incorporation of local optimization and blacklisting conformers to prevent
repeated evaluations of very similar solutions. The aim of the search is not
only to find the global minimum, but to predict all conformers within an energy
window above the global minimum. The performance of the search strategy is: (i)
evaluated for a reference data set extracted from a database with amino acid
dipeptide conformers obtained by an extensive combined force field and
first-principles search and (ii) compared to the performance of a systematic
search and a random conformer generator for the example of a drug-like ligand
with 43 atoms, 8 rotatable bonds and 1 cis/trans bond
CO Oxidation Catalysed by Pd-based Bimetallic Nanoalloys
Density functional theory based global geometry optimization has been used to
demonstrate the crucial influence of the geometry of the catalytic cluster on
the energy barriers for the CO oxidation reaction over Pd-based bimetallic
nanoalloys. We show that dramatic geometry change between the reaction
intermediates can lead to very high energy barriers and thus be prohibitive for
the whole process. This introduces challenges for both the design of new
catalysts, and theoretical methods employed. On the theory side, a careful
choice of geometric configurations of all reaction intermediates is crucial for
an adequate description of a possible reaction path. From the point of view of
the catalyst design, the cluster geometry can be controlled by adjusting the
level of interaction between the cluster and the dopant metal, as well as
between the adsorbate molecules and the catalyst cluster by mixing different
metals in a single nanoalloy particle. We show that substitution of a Pd atom
in the Pd cluster with a single Ag atom to form PdAg leads to
a potential improvement of the catalytic properties of the cluster for the CO
oxidation reaction. On the other hand, a single Au atom does not enhance the
properties of the catalyst, which is attributed to a weaker hybridization
between the cluster's constituent metals and the adsorbate molecules. Such
flexibility of properties of bimetallic nanoalloy clusters illustrates the
possibility of fine-tuning, which might be used for design of novel efficient
catalytic materials.Comment: 12 pages, 8 figure
Cooperative Caching and Transmission Design in Cluster-Centric Small Cell Networks
Wireless content caching in small cell networks (SCNs) has recently been
considered as an efficient way to reduce the traffic and the energy consumption
of the backhaul in emerging heterogeneous cellular networks (HetNets). In this
paper, we consider a cluster-centric SCN with combined design of cooperative
caching and transmission policy. Small base stations (SBSs) are grouped into
disjoint clusters, in which in-cluster cache space is utilized as an entity. We
propose a combined caching scheme where part of the available cache space is
reserved for caching the most popular content in every SBS, while the remaining
is used for cooperatively caching different partitions of the less popular
content in different SBSs, as a means to increase local content diversity.
Depending on the availability and placement of the requested content,
coordinated multipoint (CoMP) technique with either joint transmission (JT) or
parallel transmission (PT) is used to deliver content to the served user. Using
Poisson point process (PPP) for the SBS location distribution and a hexagonal
grid model for the clusters, we provide analytical results on the successful
content delivery probability of both transmission schemes for a user located at
the cluster center. Our analysis shows an inherent tradeoff between
transmission diversity and content diversity in our combined
caching-transmission design. We also study optimal cache space assignment for
two objective functions: maximization of the cache service performance and the
energy efficiency. Simulation results show that the proposed scheme achieves
performance gain by leveraging cache-level and signal-level cooperation and
adapting to the network environment and user QoS requirements.Comment: 13 pages, 10 figures, submitted for possible journal publicatio
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