OpenMX Ver. 3.6 supports both FFTW2 and FFTW3. In OpenMX Ver. 3.6, we assume FFTW3 as defaul. Then, you may link FFTW3 in your makefile as follows:
LIB = -L/usr/local/lib -fftw3 -llapack -lblas -lg2c -static
If you want to use FFTW2, you need to add '-Dfftw2' for the compile
option as follows:
CC = gcc -O3 -Dfftw2 -Dnompi -Dnoomp
LIB = -L/usr/local/lib -fftw -llapack -lblas -lg2c -static
Since the computational time for FFT employed in OpenMX is a small
fraction in the total computational time, you can use either
FFTW2 or FFTW3 without loosing significant efficiency.