code: public AtomContact getContact(Atom atom1, Atom atom2) {
  		return contacts.get(new Pair<AtomIdentifier>(
  				new AtomIdentifier(atom1.getPDBserial(),atom1.getGroup().getChainId()),
  				new AtomIdentifier(atom2.getPDBserial(),atom2.getGroup().getChainId()) ));
  	}
docs: Returns the corresponding AtomContact or null if no contact exists between the 2 given atoms
  @param atom1
  @param atom2
  @return