Thank you for downloading the analytic infall models. These models are discussed in detail in De Vries & Myers (2005). More documentation and updates are available from http://cfa-www.harvard.edu/~cdevries/analytic_infall.html . Although this paper includes all the analytic models discussed in the paper, I would restrict myself to the HILL5 and TWOLAYER6 models, which were found to be the best models. In order to fit a spectrum you need to place it in a two column text format (the first column should be velocity in kilmeters per second, and the second should be brightness temperature in Kelvins). Lines that begin with a hash mark '#' will be ignored. Use the hybrid version of the model you want to run, this will allow for some slow differential evolution minimization which will (hopefully) find the global minimum well followed by Nelder-Mead simplex minimization which will quickly find the bottom of that well. The hybrid programs of interest are "hill5_hybrid" and "twolayer6_hybrid". The arguments to both programs are: 1. input filename --- the file with your spectrum 2. frequency --- to perform the J(T) conversion. A frequency of 0 will work in units of J(T) instead of T. 3. vmin --- minimum velocity of the line profile. 4. vmax --- maximum velocity of the line profile. Be sure to get the entire line full width as well as a little baseline. This is the region over which chi-squared is calculated, so artifacts of the fit can sometimes appear outside this region. 5. population in generation --- The number of solutions to calculate each generation of the differential evolution. I usually use about 200-300 here. 6. generations per check --- The number of generations to run before checking for convergence in the differential evolution. I usually pick 300 here. 7. checks to convergence --- The number of checks to make before deciding the differential evolution algorithm has converged. I usually pick 3 to 5 here. 8. output file --- Where to place the fit and parameters. The output file will have a header line the following (a HILL 5 example is shown): # Tau: 4.46394 # Vlsr: 0.000296732 # Vin: 0.0993918 # sigma: 0.0937655 # Tpeak: 10.5371 # Attained Chisq: 0.539567 giving the parameters of the fit and Chi Squared (not a reduced chi-squared). Followed by the fit in a three column data file. The first two columns are the two columns of the input file, while the third column is the fit of the analytic model. Always check that the fit is good. Please email me at email@example.com if you have any questions or comments.