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Recently transverse susceptibility has been measured to test the quality of dispersions of magnetic particles. In this measurement, a dc magnetic field is applied to orient the particles. To monitor their microstructure, we record the transverse susceptibility (the response to a small ac magnetic field transverse to the dc field). To help interpret the susceptibility measurements, we have done extensive simulations with a realistic computer model that includes steric, magnetostatic (via a fast multipole method), hydrodynamic, and Brownian forces and torques on a system of 100 particles. We find that although a conventional noninteracting particle picture is quite useful for describing the high-field behavior of the system, at low magnetic field the structure and dynamics are complex and require a full-scale simulation in order to understand the experimental results.