rgsrmfgen creates an OGIP-compliant response matrix file as required for spectrum analysis within XSPEC. Because a certain level of compatibility between the spectrum and response matrix is required, it is typical and convenient (but not required) to create the spectrum first and provide it as in input to the response generator. The instrument's response within each incident (photon) energy bin is calculated for every dispersion channel. The size of the matrix is determined, therefore, by the size of the spectrum and the number of incident energy bins (rows) specified on the command line. A matrix of one hundred energy bins can be computed fairly quickly (about ten minutes) and is good for visual inspection, but is not good for analysis. Two hundred fifty bins is a minimum for use with rgsfluxer, and three thousand is probably acceptable for use with XSPEC. Be prepared to wait a while, and for periodic progress updates set the verbosity level to five.