Hi-

This is what the afni_proc.py command runs for residuals typically:

3dFWHMx                                                                      \
    -detrend                                                                 \
    -mask     mask_epi_anat.$subj+tlrc                                       \
    -ACF      files_ACF/out.3dFWHMx.ACF.errts.r$run.1D                       \
    errts.${subj}+tlrc"[$trs]" \
    >> blur.errts.1D

So, for example, you could use:
3dFWHMx \
-detrend \
-mask MASK_DSET \
-ACF ACF_FIT.1D \
DSET_TIME_SERIES \
>> ACF_PARAMS.1D
[/code]
... putting in the appropriate datasets. After running this, you will have ACF_FIT.1D.png, which is a picture of the fit to the data (based on running 1dplot on the also-created ACF_FIT.1D column file). The ACF_PARAMS.1D will be a commented file with 2 rows of numbers, looking like:

# old-style FWHM parameters
 0  0  0    0
# ACF model parameters for a*exp(-r*r/(2*b*b))+(1-a)*exp(-r/c) plus effective FWHM
 0.732927  3.10594  10.5977    8.04264

... where the numbers you want for the ACF fit are in that last row.

--pt