code: def _get_default_radius(site):
          
          try:
              return CovalentRadius.radius[site.specie.symbol]
          except:
              return site.specie.atomic_radius
docs: An internal method to get a "default" covalent/element radius
  
          Args:
              site: (Site)
  
          Returns:
              Covalent radius of element on site, or Atomic radius if unavailable