def id_binding_interface(ligand, receptor, interface_radius=9.0):
    """
    Identifies the binding interface of a ligand-receptor complex
    :param ligand: ProDy atom group for ligand
    :param receptor: ProDy atom group for receptor
    :param interface_radius: float representing interface radius
    :return: coordinates of ligand and receptor interfaces
    """

    lig_coords = ligand.getCoords()
    rec_coords = receptor.getCoords()

    rec_interface = receptor.select("all within {} of ligand".format(interface_radius), radius=interface_radius, ligand=lig_coords)
    lig_interface = ligand.select("all within {} of receptor".format(interface_radius), radius=interface_radius, receptor=rec_coords)

    return lig_interface, rec_interface