When you modify the code in $NWCHEM_TOP/src directory, you need to compile NWChem to make nwchem executable over the old one. NWChem consists of mamy million lines of code. This make NWChem is one of the biggest computational chemistry software. Full NWChem compilation normally takes 20 - 30 minutes, you can easily recompile NWChem within a few seconds/minutes, depending on how much you have modified the code. To do this, just use make and make link commands to install that particular modified fortran code and link a new executable.
The following is command used to recompile NWChem with the Fortran code in $NWCHEM_TOP/src/nwdft/scf_dft directory is modified.
export USE_64TO32=ycd $NWCHEM_TOP/src/nwdft/scf_dftmakecd $NWCHEM_TOP/srcmake linkls -la $NWCHEM_TOP/bin/LINUX64/nwchemP.S. export USE_64TO32=Y is needed for compilng program based on 32bit Libraries.
Rangsiman Ketkaew