9,841 research outputs found
Why Not Africa?
Various arguments have been used to explain Sub-Saharan Africa's economic decline. We find that a stress on investments in education as a prerequisite for more rapid growth is misplaced; that greater openness is far from sufficient to insure economic progress; that income inequality and urban bias are not so extreme as to foreclose prospects for more rapid growth and poverty alleviation; and that the constraints imposed by Sub-Saharan Africa's human and physical geography are not core explanations for the regions poor performance. If African countries can establish an institutional environment that enables individuals to gain the rewards of their investments, the alleged barriers to the region's growth should prove surmountable.
The Interaction of Rare Gas Atoms with Graphite Surfaces. II. Adatom–adatom Potentials
The Gordon-Kim local density method is applied to the calculation of the interaction energy of pairs of neon, argon, and krypton atoms adsorbed on the basal plane of graphite. At the adatom-surface equilibrium separation.the adatom-adatom interaction potentials are found to be from 12% to 20% more repulsive than the gas phase counterparts. At much smaller adatom-surface separations. the adatom-adatom effective potential well depth is only 3% of the gas phase well depth. It is found that the nonadditive contributions to the interaction energy are more important for the smaller rare gas adatoms than for the larger ones. This trend is observed experimentally. It is concluded that rare gas films on graphite are too mobile to be in registry with the substrate
Fourier Path Integral Monte Carlo Method for the Calculation of the Microcanonical Density of States
Using a Hubbard-Stratonovich transformation coupled with Fourier path
integral methods, expressions are derived for the numerical evaluation of the
microcanonical density of states for quantum particles obeying Boltzmann
statistics. A numerical algorithmis suggested to evaluate the quantum density
of states and illustrated on a one-dimensional model system.Comment: Journal of Chemical Physic
Computational Study of the Structure and Thermodynamic Properties of Ammonium Chloride Clusters Using a Parallel J-Walking Approach
The thermodynamic and structural properties of (NHCl) clusters,
n=3-10 are studied. Using the method of simulated annealing, the geometries of
several isomers for each cluster size are examined. Jump-walking Monte Carlo
simulations are then used to compute the constant-volume heat capacity for each
cluster size over a wide temperature range. To carry out these simulations a
new parallel algorithm is developed using the Parallel Virtual Machine (PVM)
software package. Features of the cluster potential energy surfaces, such as
energy differences among isomers and rotational barriers of the ammonium ions,
are found to play important roles in determining the shape of the heat capacity
curves.Comment: Journal of Chemical Physics, accepted for publicatio
Energy estimators for random series path-integral methods
We perform a thorough analysis on the choice of estimators for random series
path integral methods. In particular, we show that both the thermodynamic
(T-method) and the direct (H-method) energy estimators have finite variances
and are straightforward to implement. It is demonstrated that the agreement
between the T-method and the H-method estimators provides an important
consistency check on the quality of the path integral simulations. We
illustrate the behavior of the various estimators by computing the total,
kinetic, and potential energies of a molecular hydrogen cluster using three
different path integral techniques. Statistical tests are employed to validate
the sampling strategy adopted as well as to measure the performance of the
parallel random number generator utilized in the Monte Carlo simulation. Some
issues raised by previous simulations of the hydrogen cluster are clarified.Comment: 15 pages, 1 figure, 3 table
Heat capacity estimators for random series path-integral methods by finite-difference schemes
Previous heat capacity estimators used in path integral simulations either
have large variances that grow to infinity with the number of path variables or
require the evaluation of first and second order derivatives of the potential.
In the present paper, we show that the evaluation of the total energy by the
T-method estimator and of the heat capacity by the TT-method estimator can be
implemented by a finite difference scheme in a stable fashion. As such, the
variances of the resulting estimators are finite and the evaluation of the
estimators requires the potential function only. By comparison with the task of
computing the partition function, the evaluation of the estimators requires k +
1 times more calls to the potential, where k is the order of the difference
scheme employed. Quantum Monte Carlo simulations for the Ne_13 cluster
demonstrate that a second order central-difference scheme should suffice for
most applications.Comment: 11 pages, 4 figure
Changes in Earnings Differentials in the 1980s: Concordance, Convergence, Causes, and Consequences
This paper analyzes changes in U.S. earnings differentials in the 1980s between race, gender, age, and schooling groups. There are four main sets of results to report. First, the economic position of less-educated workers declined relative to the more-educated among almost all demographic groups. Education-earnings differentials clearly rose for whites, but less clearly for blacks, while employment rate differences associated with education increased more for blacks than for whites. Second, much of the change in education-earnings differentials for specific groups is attributable to measurable economic factors: to changes in the occupational or industrial structure of employment; to changes in average wages within industries; to the fall in the real value of the minimum wage and the tall in union density; and to changes in the relative growth rate of more-educated workers. Third, the earnings and employment position of white females, and to a lesser extent of black females, converged to that of white males in the 1980s, across education groups. At the same time, the economic position of more-educated black males appears to have worsened relative to their white-male counterparts. Fourth, there has been a sizable college-enrollment response to the rising relative wages of college graduates. This response suggests that education-earnings differentials may stop increasing, or even start to decline, in the near future.
Taming the rugged landscape: production, reordering, and stabilization of selected cluster inherent structures in the X_(13-n)Y_n system
We present studies of the potential energy landscape of selected binary
Lennard-Jones thirteen atom clusters. The effect of adding selected impurity
atoms to a homogeneous cluster is explored. We analyze the energy landscapes of
the studied systems using disconnectivity graphs. The required inherent
structures and transition states for the construction of disconnectivity graphs
are found by combination of conjugate gradient and eigenvector-following
methods. We show that it is possible to controllably induce new structures as
well as reorder and stabilize existing structures that are characteristic of
higher-lying minima. Moreover, it is shown that the selected structures can
have experimentally relevant lifetimes.Comment: 12 pages, 14 figures, submitted to J. Chem. Phys. Reasons for
replacing a paper: figures 2, 3, 7 and 11 did not show up correctl
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