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Disordered two-dimensional soap froth has been simulated, with a gas fraction 0 less than unity. Upon decreasing 0, it is found that the system loses its rigidity at 0^ =0.84. This value is identified with the dense random packing of hard disks. Results are presented for the variation of yield stress and shear modulus, the latter being tentatively related to elastic-network theory.

A system of Nϭ2 -4 interacting electrons in a quantum dot, with applied magnetic field, is treated using a quantum Monte Carlo method. The standard fixed-node Monte Carlo approach is modified to facilitate the inclusion of a magnetic field giving rise to a fixed-phase approximation which is introduced here. By making use of this approximation, results whose accuracy is comparable to that of exact diagonalization can be obtained. A simple set of coordinates for the four-particle problem is also used which facilitates finding a symmetrized basis.

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