89 research outputs found
Finite H2 concentrations in superfluid 4He
We have studied the solubility of molecular hydrogen in bulk liquid He at
zero temperature using the diffusion Monte Carlo method and realistic
interatomic potentials between the different species of the mixture. Around the
He equilibrium density, the H_2 molecules clusterize in liquid-like drops
blocking the existence of a uniform dilution. On the contrary, at higher
densities the cluster formation is less feasible and metastable dilute
solutions may exist.Comment: 2 pages, 2 eps figures, contribution to the LT22 Conferenc
Isotopic effects of hydrogen adsorption in carbon nanotubes
We present diffusion Monte Carlo calculations of D adsorbed inside a
narrow carbon nanotube. The 1D D equation of state is reported, and the
one-dimensional character of the adsorbed D is analyzed. The isotopic
dependence of the constitutive properties of the quantum fluid are studied by
comparing D and H. Quantum effects due to their different masses are
observed both in the energetic and the structural properties. The influence of
the interatomic potential in one-dimensional systems is also studied by
comparing the properties of D and He which have nearly the same mass
but a sizeably different potential. The physics of molecular hydrogen adsorbed
in the interstitial channels of a bundle of nanotubes is analyzed by means of
both a diffusion Monte Carlo calculation and an approximate mean field method.Comment: 17 pages, revtex, 9 ps figures, to be appear in Phys. Rev.
Quadratic diffusion Monte Carlo and pure estimators for atoms
The implementation and reliability of a quadratic diffusion Monte Carlo
method for the study of ground-state properties of atoms are discussed. We show
in the simple yet non-trivial calculation of the binding energy of the Li atom
that the method presented is effectively second-order in the time step. The
fulfilment of the expected quadratic behavior relies on some basic requirements
of the trial wave function used for importance sampling, in the context of the
fixed-node approximation. Expectation values of radial operators are calculated
by means of a pure estimation based on the forward walking methodology. It is
shown that accurate results without extrapolation errors can be obtained with a
pure algorithm that can be easily implemented in any previous diffusion Monte
Carlo program.Comment: RevTex, 20 pages, 3 figures, accepted in J. Chem. Phy
Quantum Monte Carlo simulation of a two-dimensional Bose gas
The equation of state of a homogeneous two-dimensional Bose gas is calculated
using quantum Monte Carlo methods. The low-density universal behavior is
investigated using different interatomic model potentials, both finite-ranged
and strictly repulsive and zero-ranged supporting a bound state. The condensate
fraction and the pair distribution function are calculated as a function of the
gas parameter, ranging from the dilute to the strongly correlated regime. In
the case of the zero-range pseudopotential we discuss the stability of the
gas-like state for large values of the two-dimensional scattering length, and
we calculate the critical density where the system becomes unstable against
cluster formation.Comment: 6 pages, 5 figures, 1 tabl
High-order Time Expansion Path Integral Ground State
The feasibility of path integral Monte Carlo ground state calculations with
very few beads using a high-order short-time Green's function expansion is
discussed. An explicit expression of the evolution operator which provides
dramatic enhancements in the quality of ground-state wave-functions is
examined. The efficiency of the method makes possible to remove the trial wave
function and thus obtain completely model-independent results still with a very
small number of beads. If a single iteration of the method is used to improve a
given model wave function, the result is invariably a shadow-type wave
function, whose precise content is provided by the high-order algorithm
employed.Comment: 4 page
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