For charge densities and potentials half the storage can be saved if one of this flags is used because
To use a real to complex FFT you must specify -DNGXhalf for the serial version and -DNGZhalf for the parallel version. If -DNGXhalf is specified for the serial version the real to complex FFT is "simulated" by a complex to complex FFT.
Mind: If this flag is changed in the makefile, recompile all *.F files. This can be done using touch *.F.