The RCSB_VAL_CONTACT category lists non-bonded atoms within the 
    assymetric unit of the entry that are in close contact.
    For those contacts not involving hydrogen a limit of
    2.2 Angstroms is used. For contacts involving a hydrogen atom 
    a cutoff of 1.6 Angstrom is used.
    This is a completely derived category.  Do not edit.