def find_empty_space(res,sphere,densMap,CA):
    
    """
    
    Returns list of empty voxels in voxel sphere shell
    
    Arguments:
    
       *res*
           residue where search is happening around
       *sphere*
           voxel sphere shell to be expanded around CA voxel
        *densMap*
           Solvent accessible surface (masked array)
        *CA*
           Calpha voxels
           
    """ 
    
    starters = []
    (x,y,z) = CA[res]
    for (x_s,y_s,z_s) in sphere:
        if((densMap.x_size() > (x+x_s) >= 0) and (densMap.y_size() > (y+y_s) >= 0) and (densMap.z_size() > (z+z_s) >= 0)):
            if densMap.fullMap[z_s+z][y_s+y][x_s+x] <= 0:
                starters.append([x_s+x,y_s+y,z_s+z])
    return starters