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.