A fast and accurate method for evaluating the nonrelativistic free-free Gaunt factor for hydrogenic ions

A two-dimensional Chebyshev expansion of the nonrelativistic hydrogenic free-free Gaunt factor is presented which can be evaluated extremely rapidly and which gives a maximum relative error of 0.7% over a very wide range of temperatures and frequencies. This expansion is obtained from numerical values computed primarily from the exact expressions of Karzas and Latter, augmented by certain analytic approximations that are valid in regions of the energy plane where the series given by Karzas and Latter become computationally intractable. A brief table of thermally averaged Gaunt factors is given.