We present numerical simulations of 10 planetary nebulae (PN) computed with Cloudy, a state-of-the-art code for modeling photoionized astrophysical nebulae. The primary goal of this endeavor is to study abundance enhancements of bromine, rubidium, and xenon, each of which can be produced by neutron-capture nucleosynthesis in low-mass (1-8 solar mass) giant stars that are the progenitors of PN. However, since only one or two ions of these elements are typically detected in individual PN, it is necessary to correct for the abundances of unobserved ionization states. The most accurate method for determining these corrections are with numerical models that simulate the ionization equilibrium of various elements. To this end, we have added Br, Rb, and Xe to the atomic database of Cloudy, making use of newly-determined photoionization, recombination, and collisional excitation atomic data for these elements. The modified version of Cloudy has been fully tested for stability, and reproduces standard test simulations to high accuracy. We have modeled PN in which one or more ionization states of Br, Rb, and Xe have been detected, to assess the accuracy of the new atomic data and to derive elemental abundances. The input model parameters, including central star temperature and luminosity, hydrogen density, and elemental abundances were determined via a subplex optimization routine that produced the best fit to observed emission line strengths. To derive analytic corrections to unobserved Br, Rb, and Xe ions we have initiated large grids of models (over 10,000 individual simulations per grid) that span the parameter range of densities, central star temperatures and luminosities, dust compositions, and metallicities (abundances of elements heavier than helium) encountered in PN. Our results will enable Br, Rb, and Xe abundances to be determined with unprecedented accuracy in PN, revealing new details of neutron-capture nucleosynthesis, the interior structure and mixing in low-mass giant stars, and the chemical evolution of the Universe. We gratefully acknowledge support from NSF grant AST-1412928.

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