8 research outputs found

### Multiphase density functional theory parameterization of the Gupta potential for silver and gold

The ground state energies of Ag and Au in the face-centered cubic (FCC),
body-centered cubic (BCC), simple cubic (SC) and the hypothetical diamond-like
phase, and dimer were calculated as a function of bond length using density
functional theory (DFT). These energies were then used to parameterize the
many-body Gupta potential for Ag and Au. This parameterization over several
phases of Ag and Au was performed to guarantee transferability of the
potentials and to make them appropriate for studies of related nanostructures.
Depending on the structure, the energetics of the surface atoms play a crucial
role in determining the details of the nanostructure. The accuracy of the
parameters was tested by performing a 2 ns MD simulation of a cluster of 55 Ag
atoms -- a well studied cluster of Ag, the most stable structure being the
icosahedral one. Within this time scale, the initial FCC lattice was found to
transform to the icosahedral structure at room temperature. The new set of
parameters for Ag was then used in a temperature dependent atom-by-atom
deposition of Ag nanoclusters of up to 1000 atoms. We find a deposition
temperature of 500 $\pm$50 K where low energy clusters are generated,
suggesting an optimal annealing temperature of 500 K for Ag cluster synthesis

### Single chain elasticity and thermoelasticity of polyethylene

Single-chain elasticity of polyethylene at $\theta$ point up to 90% of
stretching with respect to its contour length is computed by Monte-Carlo
simulation of an atomistic model in continuous space. The elasticity law
together with the free-energy and the internal energy variations with
stretching are found to be very well represented by the wormlike chain model up
to 65% of the chain elongation, provided the persistence length is treated as a
temperature dependent parameter. Beyond this value of elongation simple ideal
chain models are not able to describe the Monte Carlo data in a thermodynamic
consistent way. This study reinforces the use of the wormlike chain model to
interpret experimental data on the elasticity of synthetic polymers in the
finite extensibility regime, provided the chain is not yet in its fully
stretched regime. Specific solvent effects on the elasticity law and the
partition between energetic and entropic contributions to single chain
elasticity are investigated.Comment: 32 pages with 5 figures included. Accepted as a regular paper on The
Journal of Chemical Physics, August 2002. This article may be downloaded for
personal use only. Any other use requires prior permission of the author and
the American Institute of Physic

### Free energy of the Fr\"ohlich polaron in two and three dimensions

We present a novel Path Integral Monte Carlo scheme to solve the Fr\"ohlich
polaron model. At intermediate and strong electron-phonon coupling, the polaron
self-trapping is properly taken into account at the level of an effective
action obtained by a preaveraging procedure with a retarded trial action. We
compute the free energy at several couplings and temperatures in three and two
dimensions. Our results show that the accuracy of the Feynman variational upper
bound for the free energy is always better than 5% although the thermodynamics
derived from it is not correct. Our estimates of the ground state energies
demonstrate that the second cumulant correction to the variational upper bound
predicts the self energy to better than 1% at intermediate and strong coupling.Comment: RevTeX 7 pages 3 figures, revised versio

### Ab initio calculations of optical properties of silver clusters: cross-over from molecular to nanoscale behavior

Electronic and optical properties of silver clusters were calculated using
two different \textit{ab initio} approaches: 1) based on all-electron
full-potential linearized-augmented plane-wave method and 2) local basis
function pseudopotential approach. Agreement is found between the two methods
for small and intermediate sized clusters for which the former method is
limited due to its all-electron formulation. The latter, due to non-periodic
boundary conditions, is the more natural approach to simulate small clusters.
The effect of cluster size is then explored using the local basis function
approach. We find that as the cluster size increases, the electronic structure
undergoes a transition from molecular behavior to nanoparticle behavior at a
cluster size of 140 atoms (diameter $\sim 1.7$\,nm). Above this cluster size
the step-like electronic structure, evident as several features in the
imaginary part of the polarizability of all clusters smaller than
Ag$_\mathrm{147}$, gives way to a dominant plasmon peak localized at
wavelengths 350\,nm$\le\lambda\le$ 600\,nm. It is, thus, at this length-scale
that the conduction electrons' collective oscillations that are responsible for
plasmonic resonances begin to dominate the opto-electronic properties of silver
nanoclusters

### Ab initio calculations of optical properties of silver clusters: cross-over from molecular to nanoscale behavior

Electronic and optical properties of silver clusters were calculated using two different ab initio approaches: (1)Â based on all-electron full-potential linearized-augmented plane-wave method and (2)Â local basis function pseudopotential approach. Agreement is found between the two methods for small and intermediate sized clusters for which the former method is limited due to its all-electron formulation. The latter, due to non-periodic boundary conditions, is the more natural approach to simulate small clusters. The effect of cluster size is then explored using the local basis function approach. We find that as the cluster size increases, the electronic structure undergoes a transition from molecular behavior to nanoparticle behavior at a cluster size of 140 atoms (diameter ~1.7Â nm). Above this cluster size the step-like electronic structure, evident as several features in the imaginary part of the polarizability of all clusters smaller than Ag147, gives way to a dominant plasmon peak localized at wavelengths 350 nm â‰¤ Î» â‰¤ 600Â nm. It is, thus, at this length-scale that the conduction electronsâ€™ collective oscillations that are responsible for plasmonic resonances begin to dominate the opto-electronic properties of silver nanoclusters