code: public String toPDB() {
  
  		String molecId1 = getMoleculeIds().getFirst();
  		String molecId2 = getMoleculeIds().getSecond();
  
  		if (molecId2.equals(molecId1)) {
  			// if both chains are named equally we want to still named them differently in the output pdb file
  			// so that molecular viewers can handle properly the 2 chains as separate entities
  			char letter = molecId1.charAt(0);
  			if (letter!='Z' && letter!='z') {
  				molecId2 = Character.toString((char)(letter+1)); // i.e. next letter in alphabet
  			} else {
  				molecId2 = Character.toString((char)(letter-25)); //i.e. 'A' or 'a'
  			}
  		}
  
  		StringBuilder sb = new StringBuilder();
  		for (Atom atom:this.molecules.getFirst()) {
  			sb.append(FileConvert.toPDB(atom, molecId1));
  		}
  		sb.append("TER");
  		sb.append(System.getProperty("line.separator"));
  		for (Atom atom:this.molecules.getSecond()) {
  			sb.append(FileConvert.toPDB(atom,molecId2));
  		}
  		sb.append("TER");
  		sb.append(System.getProperty("line.separator"));
  		sb.append("END");
  		sb.append(System.getProperty("line.separator"));
  		return sb.toString();
  	}
docs: Return a String representing the 2 molecules of this interface in PDB format.
  If the molecule ids (i.e. chain ids) are the same for both molecules, then the second
  one will be replaced by the next letter in alphabet (or A for Z)
  @return