Comparison of two non-primitive methods for path integral simulations: Higher-order corrections vs. an effective propagator approach

Two methods are compared that are used in path integral simulations. Both methods aim to achieve faster convergence to the quantum limit than the so-called primitive algorithm (PA). One method, originally proposed by Takahashi and Imada, is based on a higher-order approximation (HOA) of the quantum mechanical density operator. The other method is based upon an effective propagator (EPr). This propagator is constructed such that it produces correctly one and two-particle imaginary time correlation functions in the limit of small densities even for finite Trotter numbers P. We discuss the conceptual differences between both methods and compare the convergence rate of both approaches. While the HOA method converges faster than the EPr approach, EPr gives surprisingly good estimates of thermal quantities already for P = 1. Despite a significant improvement with respect to PA, neither HOA nor EPr overcomes the need to increase P linearly with inverse temperature. We also derive the proper estimator for radial distribution functions for HOA based path integral simulations.Comment: 17 pages, latex, 6 postscript figure

LIFE OF HRE-3 CORE TANKS

ABS>Calculations have been made of the expected life of Zircaloy core tanks in the HRE-3 assuming 5 w/ml power density in the tank, considering possible cooling systems. If the inner surface film coefficient is l000 Btu/hr ft/sup 2/ deg F or better, a thick single-walled tank appears optimum. If the inner surface coefficient is only about 100, cooling of the core tank with either D20 or ThO/sub 2/ slurry appears desirable or even necessary. (auth