lines of tension within a nanostructure

There is not currently a way to change the precision of the gradient output. Two alternatives to modifying the source that I can think of are either using the python interface to NWChem (http://nwchemgit.github.io/index.php/Release66:Python), which should give you the gradients as python objects that you could then print to higher precision; or adding

print rtdbvalues

to your input file. This will give you the gradients to full precision, but it will also print a lot of other information to the output file as it dumps the entire contents of the runtime database to the output file.