Hi Yasser, The probe radius is also needed for the SES (solvent-excluded surface), which is the one displayed in Chimera. As Tom mentioned, the SES is where the surface of the probe goes, while the SAS is where the center of the probe goes. Both SAS and SES area calculations require and depend on the probe radius. It is only the SAS triangles for drawing the surface, the locations of the SAS surface points, that are not calculated. So if you were planning to save the surface points, you could only get the ones for the SES. This is true for the currently available Chimera. We are planning to make some changes in the surface calculations, so it may change in the future. I hope this helps, Elaine On Nov 10, 2009, at 6:49 AM, Yasser Almeida Hernández wrote:

Hi Tom... I'm actually interested in compute and save the accesible surface as a grid (like DMS). If Chimera doesn't compute the SAS surface (with MSMS code), why it ask for the probe radius in Favorites > Preferences > New Surfaces?