29 research outputs found
A diffusion Monte Carlo study of small para-Hydrogen clusters
Ground state energies and chemical potentials of parahydrogen clusters are
calculated from 3 to 40 molecules using the diffusion Monte Carlo technique
with two different pH2-pH2 interactions. This calculation improves a previous
one by the inclusion of three-body correlations in the importance sampling, by
the time step adjustement and by a better estimation of the statistical errors.
Apart from the cluster with 13 molecules, no other magic clusters are
predicted, in contrast with path integral Monte Carlo results
Correlated sampling in quantum Monte Carlo: a route to forces
In order to find the equilibrium geometries of molecules and solids and to
perform ab initio molecular dynamics, it is necessary to calculate the forces
on the nuclei. We present a correlated sampling method to efficiently calculate
numerical forces and potential energy surfaces in diffusion Monte Carlo. It
employs a novel coordinate transformation, earlier used in variational Monte
Carlo, to greatly reduce the statistical error. Results are presented for
first-row diatomic molecules.Comment: 5 pages, 2 postscript figure
The Path Integral Monte Carlo Calculation of Electronic Forces
We describe a method to evaluate electronic forces by Path Integral Monte
Carlo (PIMC). Electronic correlations, as well as thermal effects, are included
naturally in this method. For fermions, a restricted approach is used to avoid
the ``sign'' problem. The PIMC force estimator is local and has a finite
variance. We applied this method to determine the bond length of H and the
chemical reaction barrier of H+HH+H. At low
temperature, good agreement is obtained with ground state calculations. We
studied the proton-proton interaction in an electron gas as a simple model for
hydrogen impurities in metals. We calculated the force between the two protons
at two electronic densities corresponding to Na () and Al
() using a supercell with 38 electrons. The result is compared to
previous calculations. We also studied the effect of temperature on the
proton-proton interaction. At very high temperature, our result agrees with the
Debye screening of electrons. As temperature decreases, the Debye theory fails
both because of the strong degeneracy of electrons and most importantly, the
formation of electronic bound states around the protons.Comment: 18 pages, 10 figure
Phenomenology of the Lense-Thirring effect in the Solar System
Recent years have seen increasing efforts to directly measure some aspects of
the general relativistic gravitomagnetic interaction in several astronomical
scenarios in the solar system. After briefly overviewing the concept of
gravitomagnetism from a theoretical point of view, we review the performed or
proposed attempts to detect the Lense-Thirring effect affecting the orbital
motions of natural and artificial bodies in the gravitational fields of the
Sun, Earth, Mars and Jupiter. In particular, we will focus on the evaluation of
the impact of several sources of systematic uncertainties of dynamical origin
to realistically elucidate the present and future perspectives in directly
measuring such an elusive relativistic effect.Comment: LaTex, 51 pages, 14 figures, 22 tables. Invited review, to appear in
Astrophysics and Space Science (ApSS). Some uncited references in the text
now correctly quoted. One reference added. A footnote adde
The axisymmetric Boussinesq-type problem for a half-space under optimal heating of arbitrary profile
A solution of the axisymmetric Boussinesq-type problem is derived for transient thermal stresses in a half-space under heating by using the Laplace and Hankel transforms. An analytical method is developed to predict the temperature field that satisfies the prescribed mechanical conditions. Several simple shapes of punches of arbitrary profile are considered and an expression for the total load is derived to achieve penetration. The numerical results for the temperature and the total load on the punch are shown graphically
Contact problem for bonded nonhomogeneous materials under shear loading
The present paper examines the contact problem related to shearpunch through a rigid strip bonded to a nonhomogeneous medium.The nonhomogeneous medium is bonded to another nonhomogeneousmedium. The strip is perpendicular to the y-axis and parallelto the x-axis. It is assumed that there is perfect bonding atthe common plane surface of two nonhomogeneous media. UsingFourier cosine transforms, the solution of the problem is reducedto dual integral equations involving trigonometric cosinefunctions. Later on, the solution of the dual integral equationsis transformed into the solution of a system of two simultaneousFredholm integral equations of the second kind. Solvingnumerically the Fredholm integral equations of the second kind,the numerical results of resultant contact shear are obtained andgraphically displayed to demonstrate the effect of nonhomogeneityof the elastic material.Peer Reviewe
Contact problem for bonded nonhomogeneous materials under shear loading
The present paper examines the contact problem related to shear
punch through a rigid strip bonded to a nonhomogeneous medium.
The nonhomogeneous medium is bonded to another nonhomogeneous
medium. The strip is perpendicular to the y-axis and parallel
to the x-axis. It is assumed that there is perfect bonding at
the common plane surface of two nonhomogeneous media. Using
Fourier cosine transforms, the solution of the problem is reduced
to dual integral equations involving trigonometric cosine
functions. Later on, the solution of the dual integral equations
is transformed into the solution of a system of two simultaneous
Fredholm integral equations of the second kind. Solving
numerically the Fredholm integral equations of the second kind,
the numerical results of resultant contact shear are obtained and
graphically displayed to demonstrate the effect of nonhomogeneity
of the elastic material
The axisymmetric Boussinesq-type problem for a half-space under optimal heating of arbitrary profile
A solution of the axisymmetric Boussinesq-type problem is derived for transient thermal stresses in a half-space under heating by using the Laplace and Hankel transforms. An analytical method is developed to predict the temperature field that satisfies the prescribed mechanical conditions. Several simple shapes of punches of arbitrary profile are considered and an expression for the total load is derived to achieve penetration. The numerical results for the temperature and the total load on the punch are shown graphically.Peer Reviewe