The SMAC coefficients for Sentinel-2A are now available, and have been added to the CESBIO repository for SMAC coefficients. These coefficients were computed and checked by CNES colleagues in the Optical Measurement Physics service. If you happen to use these coefficients in your work, please thank CNES and CESBIO for providing them.
As already said in an earlier post :
The Simplified Model for Atmospheric Correction (SMAC) is the perfect model to perform easy, quick and not too dirty atmospheric corrections. It is based on very simple analytic formulas, based on the 5S model. The 49 coefficients of this model are fitted using a large number of radiative transfer simulations with the 6S model (the old historic version, not the recent vector version). This software is not very accurate (much less than MACCS), and it requires in-situ measurements for the aerosol optical thickness, and weather analyses for ozone and water vapour. If these data are available, in most cases, its accuracy is within 2 and 3 percent, if we do not account for adjacency effects and slope effects, and it may be worse for large viewing and solar angles (above 70°) or within strong absorption bands.
We are also providing a python code to use these coefficients. This code is described and available here.
 Rahman, H., & Dedieu, G. (1994). SMAC: a simplified method for the atmospheric correction of satellite measurements in the solar spectrum. REMOTE SENSING, 15(1), 123-143.