While our interest is focused on the big molecules in our crystals, most of
the molecules in the unit cell are small. They are the solvent molecules;
mostly water molecules. Since there are a large number of these molecules to
track, and they are created and deleted often while the model is refined,
listing RESIDUE statements for each one is cumbersome. TNT has a mechanism
where the residue type of a single residue molecule can be deduced from the
names of the atoms it contains. The deductive rules are entered on
ASSUME RESIDUE_TYPE statements (see page 
).
For single residue molecules you have the choice of either putting the proper ASSUME RESIDUE_TYPE statements in your control file or building RESIDUE statements for each occurrence in your sequence file.