Chemical and physical fractions of soil organic matter under various management regimes in Roraima, Brazil.

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Quantum Monte Carlo study of quasi-one-dimensional Bose gases

We study the behavior of quasi-one-dimensional (quasi-1d) Bose gases by Monte Carlo techniques, i.e., by the variational Monte Carlo, the diffusion Monte Carlo, and the fixed-node diffusion Monte Carlo technique. Our calculations confirm and extend our results of an earlier study [Astrakharchik et al., cond-mat/0308585]. We find that a quasi-1d Bose gas i) is well described by a 1d model Hamiltonian with contact interactions and renormalized coupling constant; ii) reaches the Tonks-Girardeau regime for a critical value of the 3d scattering length a_3d; iii) enters a unitary regime for |a_3d| -> infinity, where the properties of the gas are independent of a_3d and are similar to those of a 1d gas of hard-rods; and iv) becomes unstable against cluster formation for a critical value of the 1d gas parameter. The accuracy and implications of our results are discussed in detail.Comment: 15 pages, 9 figure

Projected multicluster model with Jastrow and linear state dependent correlations for 12≤A≤1612 \leq A \leq 16 nuclei

Variational wave functions based on a Margenau-Brink cluster model with short range and state dependent correlations, and angular momentum projection are obtained for some nuclei with $12 \leq A \leq 16$. The calculations have been carried out starting from the nucleon-nucleon interaction by using the Variational Monte Carlo method. The configuration used consists of three alpha clusters located at the apexes of an equilateral triangle, and an additional cluster, not necessarily of alpha type, forming a tetrahedron. This cluster is located at the top of its height. Short-range and state dependent correlations are included by means of a central Jastrow factor and a linear operatorial correlation factor respectively. Angular momentum projection is performed by using the Peierls-Yoccoz operators. Optimal structures are obtained for all the nuclei studied. Some aspects of our methodology have been tested by comparing with previous calculations carried out without short range correlations. The binding energy, the root mean square radius, and the one- and two-body densities are reported. The effects of correlations on both the energy and the nucleon distribution are analyzed systematically.Comment: 19 pages, 6 figure

Solidification of small para-H2 clusters at zero temperature

We have determined the ground-state energies of para-H$_2$ clusters at zero temperature using the diffusion Monte Carlo method. The liquid or solid character of each cluster is investigated by restricting the phase through the use of proper importance sampling. Our results show inhomogeneous crystallization of clusters, with alternating behavior between liquid and solid phases up to N=55. From there on, all clusters are solid. The ground-state energies in the range N=13--75 are established and the stable phase of each cluster is determined. In spite of the small differences observed between the energy of liquid and solid clusters, the corresponding density profiles are significantly different, feature that can help to solve ambiguities in the determination of the specific phase of H$_2$ clusters.Comment: 17 pages, accepted for publication in J. Phys. Chem.

Spin-orbit interaction in Hartree-Fock calculations

The contribution of the spin-orbit interaction in Hartree-Fock calculations for closed shell nuclei is studied. We obtain explicit expressions for the finite range spin-orbit force. New terms with respect to the traditional spin-orbit expressions are found. The importance of the finite-range is analyzed. Results obtained with spin-orbit terms taken from realistic interactions are presented. The effect of the spin-orbit isospin dependent terms is evaluated.Comment: To be published on Nuovo Cimento

Linearly scaling direct method for accurately inverting sparse banded matrices

In many problems in Computational Physics and Chemistry, one finds a special kind of sparse matrices, termed "banded matrices". These matrices, which are defined as having non-zero entries only within a given distance from the main diagonal, need often to be inverted in order to solve the associated linear system of equations. In this work, we introduce a new O(n) algorithm for solving such a system, being n X n the size of the matrix. We produce the analytical recursive expressions that allow to directly obtain the solution, as well as the pseudocode for its computer implementation. Moreover, we review the different options for possibly parallelizing the method, we describe the extension to deal with matrices that are banded plus a small number of non-zero entries outside the band, and we use the same ideas to produce a method for obtaining the full inverse matrix. Finally, we show that the New Algorithm is competitive, both in accuracy and in numerical efficiency, when compared to a standard method based in Gaussian elimination. We do this using sets of large random banded matrices, as well as the ones that appear when one tries to solve the 1D Poisson equation by finite differences.Comment: 24 pages, 5 figures, submitted to J. Comp. Phy

Monte Carlo Analysis of a New Interatomic Potential for He

By means of a Quadratic Diffusion Monte Carlo method we have performed a comparative analysis between the Aziz potential and a revised version of it. The results demonstrate that the new potential produces a better description of the equation of state for liquid $^4$He. In spite of the improvement in the description of derivative magnitudes of the energy, as the pressure or the compressibility, the energy per particle which comes from this new potential is lower than the experimental one. The inclusion of three-body interactions, which give a repulsive contribution to the potential energy, makes it feasible that the calculated energy comes close to the experimental result.Comment: 36 pages, LaTex, 11 PostScript figures include

High-quality variational wave functions for small 4He clusters

We report a variational calculation of ground state energies and radii for 4He_N droplets (3 \leq N \leq 40), using the atom-atom interaction HFD-B(HE). The trial wave function has a simple structure, combining two- and three-body correlation functions coming from a translationally invariant configuration-interaction description, and Jastrow-type short-range correlations. The calculated ground state energies differ by around 2% from the diffusion Monte Carlo results.Comment: 5 pages, 1 ps figure, REVTeX, submitted to Phys. Rev.

Spiked oscillators: exact solution

A procedure to obtain the eigenenergies and eigenfunctions of a quantum spiked oscillator is presented. The originality of the method lies in an adequate use of asymptotic expansions of Wronskians of algebraic solutions of the Schroedinger equation. The procedure is applied to three familiar examples of spiked oscillators