[Chimera-users] Selecting only protein surfaces programmatically

Chimera friends- I am creating a python script to take the sum of the distance from every atom in a given zone around selected residues to the surface of the protein. As part of this, I would also like to calculate the minimum distance to the surface of any given atom (based on the current surface rolling ball radius algorithm). However, I have been unable to find a command to select only the surface of a protein that works consistently across interfaces. What commands could I do this with both programmatically (in python) and in the GUI command line? Thanks! Ashton