At the 
-point half the storage for the wavefunctions can be saved
if one of these flags is used because
To use a real to complex FFT you must specify -DwNGXhalf for the serial version and -DwNGZhalf for the parallel version. If -DwNGXhalf 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.
It is a good idea to compile the -point only version in a separate
directory (for instance vasp_gamma). Copy all files
from vasp to vasp_gamma, copy makefile.machine to makefile,
and edit the makefile. Add the wNGXhalf (or wNGZhalf) flag to the cpp line.
CPP = /usr/ccs/lib/cpp -P -DNGXhalf -DwNGXhalf $*.F >$*.fUsually the
-point only version is 2 times faster than the
conventional version, but if the FFT-routines do not support
real to complex than the gain might be considerably smaller.